You Can't Lose Your Salvation

If you're a Protestant who believes in eternal security, you can't lose your salvation. But if you do become an unrepentant grave sinner that would prove you were never saved in the first place. So imagine that eight, that apostate comes back being away from God for 10 years has done terrible things. What does he need to say to God to ask for forgiveness? Can he just say the our father forgive us our trespasses?