Salam, Essie.

Thank you for your question.

Indeed, this is basically a question about the psychology of humans in general and not particularly about Muslims as such. Because, from the psychological point of view, it is the human mind that absorbs religious faith, no matter what its content may be.

Mostly it is the emotional appeal of a religion characterized by its corpus of creeds, practices, and history, rather than logic and reason, which plays a key role in the religious life of a person. 

But at the same time, it cannot be denied that logic and reason often play a strong supportive role in the matter of faith. Therefore, it is common experience that scientific discoveries, or objective research findings, have a discernible impact on thinking people, resulting in an increase or decrease of their religious faith. There are several instances that go to bear out the validity of this simple fact.

There is the case of the famous chemist, Friedrich August Kekulé, who wrote about a dream in which two snakes whirled before his eyes in a ring, that gave him a clear concept of the benzene ring, leading to this great discovery,

...I was sitting writing on my textbook, but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by the repeated visions of the kind, could now distinguish larger structures of manifold conformation; long rows sometimes more closely fitted together all twining and twisting in snake-like motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke; and this time also I spent the rest of the night in working out the consequences of the hypothesis. (Roberts 75-81)

This event in the life of the scientist can be interpreted by a religiously-oriented person as divine intervention to help mankind along the path of progress when our scientists helplessly grope in the dark. Indeed, such a view boosts a person's faith. At the same time a skeptic may offer a different interpretation of the event, claiming that his doubts about the existence of God have only increased.

Sir Fred Hoyle was one of Britain's best-known mathematicians and astronomers who spent decades searching for answers to questions of the origins of life and the origin and age of the universe. He was one who at first rejected any theory that would point to the existence of God.

But eventually, as he progressed in his studies and research, he came to believe that the fine-tuning of the universe as a whole, offered evidence for the presence of a Master-Designer. (Demme and Sarfati)

The foregoing example shows how an agnostic or atheistic scientist could gradually evolve in course of time to become a believer in God. And naturally, the study of the intellectual transformation of such scientists can lead a person to have an increase in religious faith, as it were.

Indeed, overwhelming instances of good fortune or bad fortune may often serve as turning points in the life of a person, whether Hindu, Christian, or Jew. I know the case of a religious person who claimed to have lost his faith in God following the death of his young son. And yet, the same shattering event in another person's life may have an opposite effect.

A Muslim (as well as a Jew and a Christian) in this context can remember the story of Prophet Abraham (peace be upon him): how God commanded him to sacrifice his only son and when he was prepared for it, God told him to sacrifice a lamb in the place of his son.

The foregoing story of the sacrifice of Prophet Abraham leads a religious person to have a stronger faith in God as the All-Merciful Creator and Sustainer of this universe. This is the teaching of the three scriptures of the three Semitic religions of Judaism, Christianity, and Islam.

So increase or decrease of religious faith is the common experience of Muslims as well as of the followers of other religions as well. One can see that a fluctuation or vacillation of one's faith, depending on the nature of life experiences, is quite natural and typical of human psychology, irrespective of a person's religious orientation.

In the early history of Islam, the Prophet Muhammad (peace be upon him) and his Companions were faced with unbearable persecution at the hands of the leaders of the tribe of Quraish in Makkah; and their only hope was help from Allah (God) alone. In such situations of severe trial, it was natural for many to have doubts about the nascent religion of Islam itself.

And then, like sweet water to parched throats came Allah's revelations enlivening their life and spirit. Allah the Almighty Himself mentions about this in the Quran, which Muslims believe is God's word, what means:

[Whenever there comes down a surah (chapter of the Quran), some of them say: "Which of you has had His faith increased by it?" Yea, those who believe, their faith is increased and they do rejoice.] (At-Tawbah 9:124)

This verse throws light on the natural experience of the believers to feel an increase of faith as they heard the newly revealed verses recited to them. And imagine the height of the spiritual happiness they experienced when finally Islam that began in one man, in the loneliness of a mountain cave, won over the hearts of hundreds of thousands in many lands, in the course of the life of the Prophet himself.

And today Islam is the fastest growing religion in the world. And certainly the march of Islam over the continents of the world remains still like a beacon, shining through the history of human civilization continuing to give spiritual nourishment to millions of Muslims all over the world.

I hope this answers your questions. Please keep in touch.

Salam.

Works cited:

Demme, Greg and Sarfati, Jonathan. "Big bang critic dies (Fred Hoyle)". Accessed 9 November 2009.

Roberts, Royston M. Serendipity, Accidental Discoveries in Science.  New York: John Wiley and Sons, 1989.

Useful links:

When the Light of Faith Approaches...

When the Light of Faith Approaches... (Another Answer)

Doubts About Faith

About Supplication & Strengthening Faith

Criteria for True Faith

Religion & Happiness: Any Relation?