Atheists say if they see God or some sort of miracle that defies science they will believe. However this is not true for not even the religious Pharisees believed when before their very eyes upon Jesus death many Holy men resurrected and walked the streets. Even then, they still didn’t believe and they already had faith in God. This is because our hearts are so hardened and we believe so strongly in our own argument that nobody, not even God can tell us any different.
The will of an atheist is so strong that if God did appear to him, so hardened is his heart that he’d have some way to rationalise it all and convince himself he’s had too much wine or the mushrooms he ate in college have come back to haunt him. He’s going through a psychological episode and he’ll snap out of it in a few days.
So hardened are some peoples are hearts that when they witness first hand miracles they discredit them or use an excuse of some sort not to believe and this doesn’t go for just atheists but religious people too. The Pharisees saw first hand Jesus miracles but they chose to call it the work of the devil. So hardened were their hearts that they couldn’t accept this was their Lord among them and so they used excuse after excuse for their incredulity.
So sometimes seeing is still not believing. To believe in God will always be a grace not the result of some miracle or personal encounter with Christ.