If an Atheist would be able to come to you and say - I have been there in Heaven, there was/were:
a) No God
b) No Judgment for works
c) No Angels/Demons
d) Nothing but pure nothingness which consisted out of nothing
If an Atheist would be able to come to you and say - I have just been to Heaven (lets suppose that that was his real experience) and I was given to have a look into the Book of Life. I searched the index - my name wasn't there.
Then on coming back to Earth he would be able to argue.
Atheist: That then allows me to live here on Earth in whatever manner I please.
Atheist: Yes, 100%. If my name is not there, then there is 0 chance for my salvation. I refuse to believe (obey) a God who didn't choose me to eternal life.
Problems with both "strongest proofs".
1. Both proofs are impossible, for God doesn't provide trips to Heaven neither He allows to look into the secret councils of His will.
2. He chose to bring people to faith in Christ and repentance before Him (Acts 20:21) through:
a) His kindness, goodness, patience (Rom.2:4)
b) Conversations about God public or private. Acts 9:20-22; 17:17; 18:27,28; 20:20; 28:24
Here is how I understand what has to happen to an Atheist to believe in God.
1. He has to come to a point where he no longer believes that he has an answer in himself for the misery that he experiences living in denial of God.
2. He has to come to a point where he will take the Word of God as for what it really is - the Word of the Living God, God the Creator, God of Abraham, Isaac and Jacob.
3. He has to witness both in his mind and in his flesh the glorious transformation. That has to come at the point when he will obey the Gospel of Jesus Christ. In other words when the Spirit of God will start living in the former Atheist he will no longer be one.
Those will be the only remedies for an Atheist. Other than that nothing will help. And that is up to God whom He is going to quicken from the dead.