God reveals himself to those whom he wants to. A person has to be awake enough to be aware of it. But if it is God doing the revealing there is no way the person is not going to know it. Although it is their choice if they chose to ignore it or not. Whenever I hear the argument "you can't prove God exists" I also think the logical answer, I don't need to. God already has proven his existence just by what he has made. Scripture says that his invisible qualities are clearly evident and demonstrated in his creation so that those who do not believe in God have no excuse:
"For his invisible qualities are clearly seen from the world’s creation onward, because they are perceived by the things made, even his eternal power and Godship, so that they are inexcusable."-Romans 1:20.
But God's word also tells us that not all people have faith:
"For faith is not a possession of all people."-2 Thessalonians 3:2.
And it reveals to us that true faith is only a gift granted to us by God himself:
"Each one as God has given to him a measure of faith."-Romans 12:3.
That measure of faith is granted to the person by God. And it is a part of the fruitage of God's holy spirit, so one would need to pray to God for such faith and have God's holy spirit active in their lives to have faith:
"On the other hand, the fruitage of the spirit is love, joy, peace, patience, kindness, goodness, faith, mildness, self-control. Against such things there is no law."-Galatians 5:22-23.
ETA:
Sorry I hit post early. I wanted to add one more thing.
Jesus said:
"Keep on asking, and it will be given you; keep on seeking, and you will find; keep on knocking, and it will be opened to you; 8 for everyone asking receives, and everyone seeking finds, and to everyone knocking, it will be opened. 9 Indeed, which one of you, if his son asks for bread, will hand him a stone? 10 Or if he asks for a fish, he will not hand him a serpent, will he? 11 Therefore, if you, although being wicked, know how to give good gifts to your children, how much more so will your Father who is in the heavens give good things to those asking him!"-Matthew 7:7-11.
If a person really wants true faith and a real relationship with the living God it is imperative that they ask him for wisdom and faith in prayer without doubting, and it will be given them.