I just want to know what bible verse is it that talks about God appearing to a group of non-believers and even after seeing God they still didn't believe.
Here is a text - Rom 1:18-25 that describes people who know God but refuse to obey him and thus become depraved. V21, 22 summarises this:
For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened. 22Professing to be wise, they became fools
Here are three more (radical) suggestions that answer the question but may not satisfy the question(!?!)
- Judas Iscariot met and knew Jesus (while on earth but who was still God, John 1:18, 5:17, 18, 20:28, etc) but later, betrayed Jesus and then went out and hanged himself.
- The rich young ruler, Matt 18:18-25, who went away very sad because he loved his riches more than Jesus.
- There is also the instance in Rev 6:15-17 where the wicked see Jesus (the "Lamb" on the throne) and ask to be destroyed because they do not want to be in His presence.
In all these cases, the people know and recognise God but refuse to have Him as their God.
It's not an exact fit, but you may be thinking about the parable of the Rich Man and Lazarus. It's found in the Gospel of Luke, chapter 16.
In the parable, the Rich Man dies unbelieving and goes to Hell (Hades). He asks that Lazarus (who is also dead) be sent to warn his family so that they will not suffer the same fate, since they will believe if someone returns from the dead. He is told that if his family do not believe the prophets, then they will also not believe even if someone returns from the dead.
Most Christians would take the parable as a symbolic story rather than a historical occurrence, but it does describe a supernatural appearance which is not believed.
God is a spirit, says Jesus, John 4:24. He has no visible form, of himself. Therefore if he 'appears' to a person on earth, it is to faith that he is appearing.
If a person is an unbeliever, or, as you say a 'non-believer', there is no faith there to which God may appear.
God has appeared, in manifestation, in the Person of the Son, Jesus Christ. Multitudes saw him - and many did not believe that he was whom he truly was. But some did believe. They believed that Jesus Christ - in manifest form - was the Son of God.
But many, though they saw miracles and were impressed, did not believe in Him.
God himself, the Father and the Son (who are one, for Jesus said "I and my Father are One, John 10:30) are seen by faith, not natural sight.
In Acts 9:7 and Acts 22:9 we read of men accompanying Saul of Tarsus when he was overwhelmed by a light and he heard a voice. The men also are reported to have seen the light (but were not, like Saul, blinded by it) and to have been aware of a voice but were not able to report the speech of the voice.
Only Saul, later Paul, believed in Jesus Christ because of this incident.
Perhaps this is the text in the memory of the OP.