Matthew 5:48 - Be Perfect (teleioi). The word comes from telo, end, goal, limit. Here it is the goal set before us, the absolute standard of our Heavenly Father. The word is used also for relative perfection as of adults compared with children. Absolute perfection, sinless, without fault or defect, flawless for us will not become a reality until we are present with the Lord, but that is the goal to strive for. Yet teleioi is also used to refer to the maturity of an adult.
I've seen people who teach sinless perfection also abuse and misuse the words of Jesus "go and sin no more" in John 8:11. Jesus was telling the woman to go and leave her life of sin, namely, she had been practicing adultery. Also, Jesus cannot compromise his perfect holy standard, so of course He is not going to say, "go and just sin a little bit."