As we become older and more educated, it is quite normal to question the existence of God. How do we know that God exists is a key question in life, which we all must personally answer. I clearly remember during late adolescence beginning to wonder about God's existence. I have come to personally accept that the existence of God can be deduced primarily from the 3 C's: Cosmos, Consciousness, and Conscience.
The sudden and explosive expansion of cosmic space and time at the "Big Bang" means that the universe had a beginning - and hence a Beginner. After such a violent beginning, it seems reasonable to expect that the universe would behave in a random, chaotic, and unpredictable manner. However, the universe is surprisingly well-ordered and predictable, governed by the laws of physics. This order in the cosmos points to an intelligent designer of these universal laws. It's also amazing that these physical laws can be mathematically expressed so simply and elegantly (e.g., e=mc2, F=ma). Only a creative mind could have caused this order and elegance.
Science has discovered many other evidences of the "fingerprint of God" in the universe and in life. One such evidence from physics is that the many physical constants and quantities of the universe are "finely tuned" and need to be delicately balanced "just so" as they are in order for life to exist. For example, even an infinitesimally small change in the gravitational force constant or the electromagnetic force constant would make the existence of stars like our sun, and hence life on earth, impossible. Another evidence from biology is the information encoded in the DNA molecule. An intelligent mind is the only known source of intelligible information.
The presence of human consciousness also points to the existence of God. If life had naturally evolved, as envisioned by Darwinian evolutionary theory, there would be no need for living organisms to have attainted the self-awareness and reasoning ability found in human beings. Mere instinct, which guides behavior in animals, is sufficient for survival. The consciousness found in human beings is not necessary for survival and points to something more. For example, only humans can appreciate beauty and discover the scientific principles that govern the universe. This indicates that humans are predestined for more than mere survival. Only humans have the free will that comes with consciousness. We are not robots whose actions are driven by instinct. We have the ability to think things through. We are free to decide what to do and how to do it. It's the presence of a conscious free will that separates humans from the animals.
Given that humans have a free will, we could do whatever we please. However, the morality of our actions is guided by our conscience. Why do people all over the world intuitively know the difference between right and wrong? The human conscience is not necessary for survival. Animals survive just fine without knowing right from wrong. The most compelling reason for humans to be able to know right from wrong is that we were created to be moral beings. We intuitively know the difference between right and wrong and are supposed to use our free will accordingly. This indicates that humans, unlike animals, are going to be held accountable for the actions of our free will as guided by an informed conscience.
Finally, it's normal to wonder why God doesn't more clearly reveal Himself and unquestionably prove His existence to us. It doesn't take faith to accept the existence of the sun or the moon. They are plain to see. However, God fully respects our free will. He has chosen to offer us an invitation that isn't overshadowed by coercion, such as would occur with an overpowering revelation that would leave us cowering in submission. Rather, God has given us the freedom to choose to believe in His existence - or not. Love cannot be coerced. God wants us to freely choose to know, love, and serve Him. This is our calling and gives meaning and purpose to our lives.