Doubt is not a unique Christian problem, but a human one. Some believers are even congenital doubters or have cerebral doubts that weakens their faith, but they still have faith even if the size of a mustard seed. If they seek the answers from the right sources, these doubts can be answered. Doubt is not the opposite of faith, but an element of it. They can and do co-exist.
Faith is putting into action and trusting in what you do believe. Faith is knowledge in action, not having all the answers. But faith is not inert but grows for the believer and he will find out that God will eventually answer his questions. There is a doubt-faith continuum that we all fall on. Perfect certitude is not necessary for faith and that’s why it’s called faith. But saving faith is the gift of God and we find God through this grace, not our works.
We cannot believe in God in our own intellectual powers or wisdom; we need to be enlightened by God who opens our hearts and kindles faith within us There is mundane faith in God for the things of the world, using our spiritual gifts, and doing His will and then there’s saving faith: Saving faith is the gift of God but God can rebuke us for showing little or no faith in doing His will, as it is written, “O, ye of little faith… why did you doubt?” “I believe, help thou mine unbelief.”
There’s hope! Jesus can strengthen our faith and we are to pray for this. But note that our faith must be tried as if by fire and be tested to see if it’s genuine and not bogus. This is the good news: Christ doesn’t require perfect faith, but only unfeigned and sincere faith that isn’t hypocritical (cf. 1 Tim. 1:5; 2 Tim. 1:5). Soli Deo Gloria!