So how can anyone be certain about anything? The answer is simple. You can't be certain. When you encounter someone that tells you they know the answer to life's questions, absolutely, for certain -- RUN! They're taking the lazy path. They're not certain -- they have just stopped asking questions and are now expecting you to do likewise.
Look at these responses, quoted from some of the world's great thinkers regarding the issue of certainty:
"Certainty? In this world nothing is certain but death and taxes." - Benjamin Franklin
"The quest for certainty blocks the search for meaning. Uncertainty is the very condition to impel man to unfold his powers." - Erich Fromm
"Doubt is not a pleasant condition, but certainty is absurd." - Voltaire
"What men really want is not knowledge but certainty." - Bertrand Russell
"Fear comes from uncertainty. When we are absolutely certain, whether of our worth or worthlessness, we are almost impervious to fear." - William Congreve
One of the most unfortunate consequences of certainty is that it leads to the soft insulation of belief. Yes, for a spiritual person there are things in life which should not simply be taken by faith and be believed without question. God made us with inquisitive minds and intended for us to use them. When we do not question and we become certain about everything, I believe we in fact disappoint God. I believe faith and belief are for trusting in God, not for being lazy with our minds.
If we allow our minds to soften into pudding by allowing belief and faith to intrude into areas never intended to be handled by faith and belief, we become certain something is true and must therefore protect that conclusion from falsification. For example, articles of faith such as the exact method used by God to perform the Creation are viewed by us as something we must protect from falsification, and we therefore attempt to disprove well-grounded scientific theories such as Evolution. Starting with the conclusion, then proclaiming to be false anything that calls our conclusion into question is NOT how scientific discoveries are made.
I believe that faith and reason are not in conflict, and where the question of Creation and Evolution are involved, I believe God respects those who can say that we believe God created the earth, and that science proved life evolves over time. I believe in my heart both things can be true and there is no conflict. I do not need to see proof of the Creation to know it is true. For me, proof is for science, not for the realm of the divine. My proof of God is in my heart. I need no further proof of God.
I believe God wants us to explore, to study, to question the things we see around us. God is strong and powerful and he can take a few 'Why' questions from his questions just like we can handle them from our 3-year-old grandchildren. Don't be afraid to ask questions. Questions are the beginning of understanding. Certainty is the end of it.