Religion has claimed that belief in God is natural. What is meant by "natural"? It does not mean we are compelled by an irresistible force to intellectually accept the existence of God; rather, we still have a choice based on how we understand the evidence. Instead, "natural" means that we incline gracefully towards a certain conclusion or action, and to resist would take added force and energy.
For example, it is natural to desire food and as such it is natural to make the decision to eat, because there exist natural inner urges that push us to do so. We may decide to control this appetite in order to avoid excess or over-eating, but it is difficult and harmful to ignore this natural urge altogether. The same may be claimed of every other natural inclination, whether it is the desire for water, attraction towards the opposite gender, purpose and enjoyment to name a few. Closing your eyes to your inner impulses is like swimming upstream instead of along with the current.
Religious scholars and thinkers also tend to believe in a natural propensity to believe in spiritual truths, like those of angels, Satan, and God. This propensity colors our perception, often causing us to see deeper than what our senses reveal at the surface – revealing purpose and meaning in all aspects of the world and within. This inner metaphysical propensity tends to cause the feeling of life and purpose in even the most mundane aspects of life. How often do we see living things in the clouds above? Or pictures in our bathroom tiles? We internalize these sparks as hints of life beyond what we perceive with our senses.
Religious scholars describe this propensity as a starting point for belief in God and in other spiritual realities. Just as food and sexual urges should not be shunned, this too may be ignored only to our own detriment. According to Muslim scholars, denial of God naturally leads to the "hunger pangs" of uncertainty – pangs that eat away at the soul and bring sadness to the heart.
One may question whether inner propensities accurately point us to external realities. Does an inner divine feeling mean God exists? A baby's cries seem to be the roadmap to a mother's milk, thirst sets us off in the direction of water, and our loneliness and longing for companionship leads towards our fellow human beings and future spouses. All those urges point to something real. Allama Tabatabai says, "If such [an inner] wish were not to have an objective existence it would never have been imprinted upon man's inner nature, in the same way that if there were not food there would have been no hunger. Or if there were to be no water there would be no thirst and if there were to be no reproduction there would have been no sexual attraction between the sexes." 
Of course, inner pushes are not direct and rational proof of God's actual existence. There is no doubt, however, that they are a strong foundation for the possibility of something real and they provoke deep curiosity. All our inclinations may point to a reciprocal reality, and it is presumptuous to assume the impossibility. Thus, the inner-force of our spiritual inclination is enough reason to soul-search and research whether there is a God or not.
This cannot be too different than the inner push to search for water when thirsty in a desert. Your soul has desired it and is hopeful for its existence. Failing to begin the search is certain to lead you nowhere except deprivation and death. To search means to be hopeful in the reality of what you desire within. The longing can be so strong that it even attempts to define your purpose.
There is a reason why this type of soul-searching is sung about in songs, written about in classical literature, and depicted in movies. To assume at the onset that such a journey leads to nowhere is to deny the calling of one's own inner voice. It does not matter if you do not find what you look for or still remain uncertain – you are striving towards your end on a noble quest for satiating that inner requirement.
The modern, scientifically-minded individual who may be uncertain about religious conclusions has enough evidence to take seriously those spiritual urges as well, as a consensus develops in scientific circles that the human mind naturally develops the idea of God (though keep in mind that some scientists use this as proof that the human mind is whimsical, rather than as a reason to search for God in the real world).
Dr. Justin Barrett of the University of Oxford's Centre for Anthropology and Mind told BBC Radio 4's "Today" program: "The preponderance of scientific evidence for the past 10 years or so has shown … a predisposition to see the natural world as designed and purposeful and that some kind of intelligent being is behind that purpose. If we threw a handful [of people] on an island and they raised themselves I think they would believe in God." 
Dr. Olivera Petrovich, also of Oxford University, believes that infants are hard-wired to believe in God. She says, "Atheism is definitely an acquired position." 
Bruce Hood, professor of developmental psychology at Bristol University, says "Our research shows children have a natural, intuitive way of reasoning that leads them to all kinds of supernatural beliefs about how the world works." 
Professor Pascal Boyer, an anthropologist at Washington University, says "Religious thinking seems to be the path of least resistance for our cognitive systems. By contrast, disbelief is generally the work of deliberate, effortful work against our natural cognitive dispositions — hardly the easiest ideology to propagate." 
Such research questions the long-standing position many atheists have taken, claiming that belief is the result of indoctrination or family rearing. Of course this does not mean that doubts regarding belief are unfounded. Spiritual inclinations are a start of the journey; they are not necessarily sufficient rational proofs. Martyr Murtadha Mutahhari explains, "However, it is worth noting that we do not mean that, as the monotheistic tendency is natural and innate (fitrawii), no questions arise when the issue is dealt with at the intellectual and philosophical level. This is certainly not meant. This matter is just like every other issue that naturally—despite affirmation by natural instinct—gives rise to questions, objections and doubts in the mind of a beginner when posed at the rational level, and satisfying answers to them are also available at that level." 
So listen deep within. Do not conclude exactly what your soul is telling you without logical or scientific evidence. But when you hear a longing voice, certainly head in its direction to see what it may be.
"We will soon show them Our signs in the Universe and in their own souls, until it will become quite clear to them that it is the truth. Is it not sufficient as regards your Lord that He is a witness over all things?" (Holy Qur'an 41:53)
The analogy claiming that the inclination for food and other similar desires are similar to natural spiritual inclinations may give rise to a number of questions. One such question could be: we know that it is a mistake to over-indulge in food and sexuality – would that then mean it is bad to over-indulge in spiritual matters? Are we saying we should not worry about God too much?
It is a great question. However, spiritual and metaphysical matters are not typically liable to quantity, time, or duration or other such temporal aspects, thus making it problematic to compare them in an exact manner to food and sexuality (which happen to exist on the physical plane of existence). Though we have pointed to the similarities earlier, there is certainly a difference between metaphysical inclinations and physical inclinations – even if the differences are not entirely fundamental.
Second, it may be admitted that over indulgence in spiritual matters could be a bad thing, but let's correctly frame our understanding of "spiritual matters" first. If one is praying and fasting all day and night to the exclusion of other responsibilities, particularly worldly responsibilities, then perhaps we can say that this type of spiritual over-indulgence is a bad thing. But in reality, this is not really being "spiritual" nor is it being "Godly." Our God created the physical plane of existence as well as the metaphysical and spiritual realms, all of which correspond to our lives. To be a Godly person then would be to balance prayer, fasting, social responsibilities, family, and all other aspects of life. To put it more profoundly, it is all spiritual.
But the inner inclination to gravitate towards God needs to be carefully nurtured and carefully responded to, just like our need for food (too much or too little would be harmful). Over-indulgence in prayer can make some people hate prayer. The Messenger of Allah (peace be upon him and his progeny) said: "O Ali, this religion is firm, get into it mildly, and do not cause yourself to hate worshiping your Lord, otherwise, you will be like the one whose mount was too tired to go on, so he neither finished the journey nor preserved the mount." 
We can also recall the story of the man whom the Prophet censured because he left his family to go worship on a mountain for days on end. This, of course, does not demean the value of prayer; indeed, prayer is still the beloved fruit of the believer. Rather, Islam elevates other responsibilities even as they relate to the worldly life, and gives them a spiritual character. 
"Seek the gains of the life to come through your wealth without ignoring your share of this life. Do favors to others just as God has done favors to you. Do not commit evil in the land for God does not love the evil-doers." (Holy Qur'an 28:77)
 Shi'a by Allamah Tabatabai
 "Children are born believers in God, academic claims" in Telegraph
 "Infants 'have natural belief in God'" in The Age
 "We are born to believe in God" in The Sunday Times
 The Causes Responsible for Materialist Tendencies in the West, Part I by Martyr Mutahhari
 Disciplines of Prayer by Imam Khomeini
 For further reading on this topic, see Martyr Mutahhari's Glimpses of Nahjul Balagha: The World and Worldliness