When can we truly say a person is no longer a Christian? Is it few months of unrepentant sin or few years of repentant sin or when they say they don’t believe anymore ?
There is no clear answer to your question. The writer to the Hebrews first stated the
elementary principles of Christ, meaning those things that make a person a Christian, then told of the fate of those who
fall away, without defining precisely what
falling away is:
Heb 6:1 Therefore, leaving the discussion of the elementary principles of Christ, let us go on to perfection, not laying again the foundation of repentance from dead works and of faith toward God,
Heb 6:2 of the doctrine of baptisms, of laying on of hands, of resurrection of the dead, and of eternal judgment.
Heb 6:4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit,
Heb 6:5 and have tasted the good word of God and the powers of the age to come,
Heb 6:6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.
Heb 6:7 For the earth which drinks in the rain that often comes upon it, and bears herbs useful for those by whom it is cultivated, receives blessing from God;
Heb 6:8 but if it bears thorns and briers, it is rejected and near to being cursed, whose end is to be burned.
This is all believed to refer to early Jewish Christians who had forsaken the faith to return to Judaism. But there is no reason to believe that it doesn't also apply to an atheist returning to disbelief or a Mithraist returning to Mithraism.