Because the Bible says if you go back to the world in unbelief you no longer have the benefit of Christ's sacrifice for justification.
Perhaps all people really want to argue (with the exception of hypergracers's of course) is that this is true, but true believers will never go back to the world in unbelief so you don't have to worry about what will happen if you go back to unbelief. Well, we can see clearly from the Bible that Paul DID worry about that happening. He did not take the Hebrew church, the Galatians, and the Corinthians falling away from trust in Christ very well as if it's impossible for that to happen.
Perhaps all people really want to argue (with the exception of hypergracers's of course) is that this is true, but true believers will never go back to the world in unbelief so you don't have to worry about what will happen if you go back to unbelief. Well, we can see clearly from the Bible that Paul DID worry about that happening. He did not take the Hebrew church, the Galatians, and the Corinthians falling away from trust in Christ very well as if it's impossible for that to happen.
Additionally, The Bile cautions us....."sacrifice for forgiveness (Christ death on the Cross) is not available "......where sin continues.