The biblical challenge to an unsaved person is, “Do you know the Lord?” If someone is a “backslider” (a false convert), ask him, “Did you know the Lord?” That puts him between a rock and a hard place. If he says he did, then he is admitting that God is real and that he is in rebellion. If he says that he didn’t know Him, he is admitting that he was never a Christian. He had a false conversion, as so clearly taught in the Parable of the Sower (Mark 4). See John 17:3 for the definition of a Christian.
Judges 2:10When all that generation had been gathered to their fathers, another generation arose after them who did not know the Lord nor the work which He had done for Israel.