ANSWER: There is no line to draw.

EXPLANATION:

God sent Jesus to die for our sins, right?

ALL of our sins.

Past, present, AND future.

Even if you're a Christian, you still sin. You sin ALL THE TIME. You sin without even realizing it. Every time you think about something wrong, say something bad, tell a "little white" lie, or even something as "small" as cheating on a test or on your taxes, or even speeding when no one's around to see it, you're sinning.

Someone at my church is famous for saying, "There are no big shots or little shots. We just all need to BE shot." There are no big sins or little sins. Just sin, and we all need forgiveness, even after salvation.

God has already forgiven you of everything you're going to do, but when you ask Him for forgiveness, you're accepting the forgiveness that has already been offered to you.

"I give them eternal life and they will never perish - ever! No one will snatch them out of My hand." (John 10:28)

There is literally no possible way that you can be un-saved, or become a non-Christian after salvation. God has rescued you from Hell, and even though you may still be oppressed by Satan and the world and you may still sin all the time, God's got His hand on you. He's got you. There's no way you can ever, EVER be un-saved.