I believe Jesus sacrifice covers all sins, if a person is in faith and then leaves faith and then comes back Jesus would welcome them with open arms, but then again, how likely is that to happen. Jesus has paid the penalty for all sins we have and will commit but at the same time if we wilfully sin again and again then we will reap the consequences. Its like King David and Bathsheba, he was forgiven for his wilful sin but paid the consequences he lived a life full of family problems and enemies, i think he is an inspiration actually, he remained in faith till the end.What I have said may be obvious to many, but all the time I hear excuses from Christians to justify their sinning:
"Nobody is perfect" - nobody has said that you are
"God will forgive me" - actually, according to Matt 7:21, if you continue to sin after accepting Jesus, Jesus' sacrifice won't cover your sin (verse above)
"We're human, and humans are fallible" - typical excuse!
"It is impossible not to sin, as we are not perfect" - this article sums it up very well: https://www.gotquestions.org/go-and-sin-no-more.html
Have you heard these kind of excuses too?