The voice of one crying "In the wilderness, prepare the way of the Lord; make his paths straight..." (Isaiah 40:3 and Matthew 3:3) The original text in Isaiah is clear that the voice isn't crying in the wilderness, but is saying to straighten the paths in the wilderness.

How does one prepare the way of the Lord? By repenting from sin. It isn't that the Messiah can't come back until we repent, but that those who do not repent will be crushed or pushed aside at His arrival. (Matthew 3:3)

"Repent" doesn't mean to regret. It doesn't mean to be sinless either. To repent is to stop knowingly and willfully sinning. It means to change behavior. "Repent for the Kingdom of Heaven is at hand." (Matthew 3:2)