Rational thought is an illusion. Everyone has faith, it's just a matter of what they have faith in. Atheists have faith that there is no such thing as faith, for instance. It takes faith to breathe air, as you have to believe there is air to breathe. You can say, well I know there will be air because there was air before, so it's rational thought and not faith. This is an illusion, however, and would only be possible in a vacuum where time doesn't pass and things don't change. Therefore, due to the changing circumstances of life, the fact that there is no time but the present, and no guarantee that the present will mimic the past or the future, the present, faith is required to do anything. The question is, what do you place your faith in. Ultimately, it is wisest to place your faith in God because God is the creator of anything else one could put faith in.
From I don't see how God could plausibly exist (Christian definition of God)
Faith is actually not some mystical voodoo spiritual feeling. One's faith is the nature of that person's agreement with God. "Faith is the assurance of things hoped for, the evidence of things not seen. (Hebrews 11:1)" This verse is often misunderstood... with knowledge of God's promise through Jesus Christ, one has an ability to be assured of the hope sought for which produces evidence despite that thing having yet to happen. Let's say you have faith that the Amtrak will arrive on time at 7:05... your faith produces a confidence in that event, and that confidence produces evidence because knowing it will be on time means you'll be sure to be there by 7:05, and sure enough your faith is the reason you'll be riding the train at 7:10. Unlike fallible things we're accustomed to, God is infallible. In fact, God is infallible unto Himself, i.e. we "swear to God", but God swears by Himself, He cannot lie and there is no deception or darkness in Him. This is why Jesus would tell people "your faith has made you well"... He was not saying their mystical voodoo feeling is what cured them, no, God cured them... however, He was informing them that as far as their participation in the matter is concerned, it was their agreement to God's ability which allowed the healing to occur.
God made the heart for worship by faith... you can deny this but inevitably all anyone can and will do is worship something or someone by faith and attempt to have life through that relation.