He has written The Law on the hearts of all men...or He would be unjust to hold all men accountable!
2Co 6:14 Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness?
Unbelievers are lawless. They don't have God's laws written in their hearts. That comes through repentance and the giving of God's holy spirit.
All men will be held accountable, but not everyone is called to have God's spirit at this time. Only Christians get their calling during their lifetime and only Christians are judged during their lifetimes. The rest will be called in the general resurrection mentioned in Revelation 20 and will be judged then.