In the name of Allah, the Gracious, the Merciful
Knowledge of God’s existence is often taken for granted by believers. The authentic religious experience—affirmed again and again in a Muslim’s daily life—makes faith in God feel so natural as to be assumed. But belief in God and the quest for existential truth is not an easy prospect for many people, especially in a social environment in which faith is derided as superstition, wishful thinking, or even as a dangerous fantasy.
In the Islamic tradition, the case for God’s existence is solid in terms of its rational foundations as well as the purpose, meaning, comfort, and guidance that it gives to our lives. The Quran inspires conviction by appealing to the aspects of the inner life of human beings, namely, to the heart and the mind. Intuition and experience work in tandem with logic and reason to arrive at a state of certainty in faith.
This understanding of conviction is reinforced by modern scientific conceptions. Cognitive scientist Justin Barrett, for example, demonstrates that belief in God—and beliefs more generally—are formed and attained in two ways: 1) non-reflective, intuitive beliefs that result from experience; and 2) reflective, conscious beliefs that result from thought.[1] The human being naturally forms beliefs from these two sources. Similarly, the case for God’s existence in the Quran and Sunnah involves both sources of beliefs: heart-based appeals based on intuition and mind-based appeals based on rational reflection.

Appealing to the Heart, Intuition, and Experience

Natural Instinct – Fiṭrat Allāh

Human beings sense the existence of God—or what they perceive as a higher power—by pure instinct, with or without a prophetic revelation to guide them. Expressions of this sensus divinitatis have appeared in cultures and religions all over the world, despite them being widely separated by time, geography, and language. In Islamic spiritual terms, this is because God took a primordial covenant with every person before the world was created that they would recognize their Creator.
Allah said:

[Prophet], when your Lord took out the offspring from the loins of the Children of Adam and made them bear witness about themselves, He said, ‘Am I not your Lord?’ and they replied, ‘Yes, we bear witness.’ So you cannot say on the Day of Resurrection, ‘We were not aware of this.’[2]

Al-Suddī (d. 745) commented on this verse, saying, “For this reason, there is no one on the face of the earth but that he knows his Lord is Allah, and no one associates idols with Him except that he will say ‘I found my forefathers following another religion.’”[3] The primordial covenant results in the innate impulse within people to seek out the higher power that they can sense, as they have done in some form or another throughout all of recorded history, to the point that some scientists today argue that belief in God or a higher power is hardwired into our genes.[4]
All true and revealed religion confirms and conforms to the human nature that the Creator instilled within us. The Quran refers to human religious nature as fiṭrat Allāh, the instinctive and inherent disposition with which God created people.
Allah said:

So [Prophet Muhammad] as a man of pure faith, stand firm and true in your devotion to the religion. This is the natural disposition God instilled in mankindthere is no altering God’s creationand this is the right religion, though most people do not realize it.[5]

All people were born to be believers in God, and the revelations of the Prophets simply awaken and reinforce the disposition that is already inside us. Even polytheists, who believe in multiple gods and deities, often believe that there is an even higher power, an even greater God over all of them.
Allah said:

If you asked them who created the heavens and earth, they are sure to say, ‘God.’ Say, ‘Praise belongs to God,’ but most of them do not understand.[6]

And Allah said:

If you [Prophet Muhammad] ask them who created them they are sure to say, ‘God,’ so why are they deluded?[7]

Since all humans can sense a higher power, they will instinctively turn to God in times of danger. Every person, at some point in his or her life, will have an intense experience that causes a natural reaction to turn to prayer. There are many cases of people who experience a sudden brush with death that causes them to become more faithful to their religious tradition.

Allah said:

Whenever they go on board a ship they call on God, and dedicate their faith to Him alone, but once He has delivered them safely back to land, see how they ascribe partners to Him![8]

And Allah said:

Say [Prophet Muhammad], ‘Who is it that saves you from the dark depths of land and sea when you humbly and secretly call to Him [and say], ‘If He rescues us from this, we should truly be thankful’? Say, ‘God rescues you from this and every distress; yet you still worship others besides him.’[9]

And Allah said:

When something bad happens to people, they cry to their Lord and turn to Him for help, but no sooner does He let them taste His blessing then—lo and behold!—some of them ascribe partners to their Lord, showing no gratitude for what We have given them.[10]

And Allah said:

When man suffers some affliction, he prays to his Lord and turns to Him, but once he has been granted a favor from God, he forgets the One he had been praying to and sets up rivals to God, to make others stray from His path.[11]

Al-Ghazālī (d. 1111), in his autobiography, describes the innate religious disposition—that he himself experienced—as an urge for people to “seek knowledge of the reality of things.”[12] He describes this spiritual longing for truth as a “thirst” (ta’aṭṭash) that must be quenched. It is a void and emptiness in the heart, an uneasiness with existence, which must be filled by discovering meaning and purpose to life. And while reason is an important tool to achieve faith in God, it is still merely one step along the spiritual path: “Beyond the level of reason lies another level that opens another eye by which one gains insight into the Unseen and into what will occur in the future, among other things.”[13] 
Knowledge of God resides primarily in the heart and is reinforced by reason, but it is not achieved and consolidated by reason alone. There must be an authentic religious experience, the acknowledgment of greater purpose and meaning to the universe, and the taste of spiritual fruits. Hence, the early scholars of Islam defined knowledge of God not as a collection of facts and arguments, but rather as a light within the heart. Imām Mālik (d. 795) said, “Knowledge is not knowing many sayings. Verily, knowledge is only a light that Allah places in the hearts.”[14] And Ibn Rajab (d. 1393) said, “Knowledge is not memorizing many narrations or sayings, but rather it is a light that Allah places in the heart by which a servant understands the truth and he distinguishes it from falsehood.”[15] 

The Experience of Peace and Contentment

The “light” of the knowledge of God is found in the guidance, spiritual direction, and the sense of peace that believers attain by practicing Islam. It can fill the spiritual void in all of us. The greatest proof for the existence and Lordship of Allah, then, is discovered in the visceral experiences of the heart through which the believers find comfort, inner-peace, moral education, and meaning in life—the spiritual fruits of true religion.
Allah said:

A light has now come to you from God, and a Scripture making things clear, with which God guides to the ways of peace those who follow what pleases Him, bringing them from darkness out into light, by His will, and guiding them to a straight path.[16]

And Allah said:

Believers, be mindful of God and have faith in His Messenger: He will give you a double share of His mercy; He will provide a light to help you walk; He will forgive you— God is most forgiving, most merciful.[17]

And Allah said:

Truly, it is in the remembrance of God that hearts find peace.[18]

The Quran promises those who have faith that they will live a “good life” and move into an infinitely greater existence in the afterlife. The peace that believers experience in this life is only a brief taste of the peace to come in the Hereafter. Reflecting upon this fact day after day in prayer contributes to a state of calm and tranquility.
Allah said:

To whoever, male or female, does good deeds and has faith, We shall give a good life and reward them according to the best of their actions.[19]

And Allah said:

[But] you, soul at peace: return to your Lord well pleased and well pleasing; go in among My servants; and into My Garden.[20]

This is not a promise that believers will never experience adversity. Indeed, there will come times—Allah guarantees—in which the believers will be tested for their patience and perseverance. Rather, Allah promises that believers, through their spiritual practices and moral and theological education, will achieve a stable state of contentment and satisfaction with life.
Allah said:

Your Lord is sure to give you so much that you will be well satisfied.[21]

And the Prophet ﷺ said:

He has succeeded who embraces Islam, whose provision is sufficient, and who is content with what Allah has given him.[22]

Prophet Muhammad ﷺ described spiritual contentment as the “sweetness of faith,” a fruit of true religion that is to be tasted and experienced. It involves authentic encounters with the Divine by which a believer becomes grounded and certain in the truth of his or her faith.
The Prophet ﷺ said:

He has tasted the sweetness of faith who is content with Allah as his Lord, Islam as his religion, and Muhammad as his prophet.[23]

The contentment described in Islamic texts consists of a stability of being that protects believers from oscillating between extremes of momentary euphoria and sorrow, such that they can remain in a state of calm and tranquility regardless of what is happening in their lives. It is manifested in a detachment from worldly possessions and temporal concerns (zuhd), and an end to the fear of poverty. This spiritual state of being is, in fact, what the Prophet ﷺ described as true wealth.
The Prophet ﷺ said:

Wealth is not in having many possessions. Rather, true wealth is the richness of the soul.[24]

And the Prophet ﷺ said:

Wealth is in the heart and poverty is in the heart. Whoever is wealthy in his heart will not be harmed no matter what happens in the world. Whoever is impoverished in his heart will not be satisfied no matter how much he has in the world. Verily, he will only be harmed by the greed of his own soul.[25]

Consequently, those who achieve this state of being—along with their basic worldly needs—are truly the wealthiest people in the world.
The Prophet ﷺ said:

Whoever among you wakes up secure in his property, healthy in his body, and he has his food for the day, it is as if he were given the entire world.[26]

When people are consumed by wealth, possessions, and worldly status, they will inevitably experience a void that leads to unhappiness no matter how much they have. The early Muslims used to say, “If the kings of the world and their sons knew what we have of blessings and happiness, they would fight us for it with their swords.”[27] True and stable happiness is not something that we acquire “out there,” as kings and powerful leaders might imagine. The material world cannot make us happy and content for long; we have to discover the means of happiness within the heart through our relationship with the Creator. It is the free gift of Islam for all.
In this state of contentment, believers are better equipped to endure the inevitable vicissitudes of life. Hypocrites, those who are insincere in their religion, by contrast, are unable to pass through adversity with the same sense of stability and calm.
The Prophet ﷺ said:

The parable of the believer is that of a crop which withstands the wind, for the believer continues to withstand the suffering of trials. The parable of the hypocrite is that of a cedar tree, for it does not budge until it is uprooted.[28]

By tasting these spiritual fruits, the believers strengthen their conviction with every positive and meaningful experience. Knowledge of God is consolidated through these experiences much more effectively than through logical or philosophical argument.
Al-Ghazālī writes:

Hence, when you have understood the meaning of prophecy and you have abundantly examined the Quran and traditions, you will inevitably arrive at the knowledge that [Muhammad]—peace and blessings of God be upon him—embodied the highest levels of prophecy, and that is reinforced by experiencing his teachings in regards to worship and its effects on purifying the heart… Thus, when you experience that one thousand, two thousand, and many thousands of times over, you will necessarily attain knowledge in which there is no doubt.[29]

One reason this is the most effective method to strengthen faith, as demonstrated by cognitive scientists, is that the intensity of religious experiences makes them unforgettable.[30] A believer who witnesses the benefits of spiritual practice in his or her lifeday after daywill attain a level of certainty and tranquility that they would never consider abandoning.

An Abandoned Instinct

Not everyone achieves an authentic religious experience or understands it in a proper theological framework. Human nature is not immune to alteration by misguided doctrines and philosophies that contradict our healthy instincts, even ideas that claim a basis in theism.
The Prophet ﷺ said:

No one is born except upon natural instinct. Then his parents turn him into a Jew or a Christian or a Magian. As an animal produces their young with perfect limbs, do you see anything defective?[31]

In this tradition, the Prophet ﷺ tells us that everyone is born to accept the simple, natural worship of one unique, powerful, and benevolent Creator. However, for various reasons, people invent religious doctrines that anthropomorphize God (ascribe human features to Him), or ascribe divine attributes to created beings (such as deifying saints and sages), or deny the Creator altogether.
When human nature and intuitive belief in one God are corrupted, either by negative experiences or the influence of others, then logical arguments may be required for a person to return back to his or her natural state.
Ibn Taymiyyah (d. 1328) writes:

The establishment and recognition of the Creator is a fundamental instinct in human nature, even though some people have done something to corrupt their nature such that they need an argument to achieve knowledge of God. This is the opinion of the majority of people, as well as the skilled debaters, that knowledge of God is sometimes achieved effortlessly and other times by argument, as was recognized by more than one of the theologians.[32]

Our intuitive beliefs, which we gain from our natural disposition and experiences, serve to anchor and inform the beliefs that we gain through reflection and conscious, rational deliberation.[33] In this way, reading and reflecting on the Quran builds upon intuitive experiences of the heart by appealing to the mind and the power of reason.

Appealing to the Mind, Reason, and Logic