The Greek word telios, often translated as perfection, actually means completeness. Paul wasn't saying Christians should be sinless, but rather wholehearted in their devotion to God and honest about the sin they do have, finishing the spiritual race without hidden resentment.
The word perfection is the Greek word telios, and it doesn't mean without sin in certain contexts. It means complete. So Paul says when he's talking about finishing the race, that he's on this journey, he's about to die, but he then ends up saying, I have become perfect. But he wasn't saying without sin. He was saying, I've completed the race. I've put all my heart on the line. I've revealed to God my heart every day. There's nothing in there that I know of that I've allowed to fester. I have finished the race. I've died for the King.
