I know this is a deep question. And it can go in many different directions.
For one, there will always be one recognized sin by individuals who do not believe in sin and that sin is intolerance. Most morel relativists would argue that intolerance is morally unacceptable.
In reality, even if one were to hold that there is no such thing as sin or that morality is subjective, it does not make it so.
Then there is the theological concept of culpability. If every single human being on the planet were convinced that there was no such thing as sin, would there be a need for our Savior?
For the record, I believe in sin. I believe I am a sinner and “I confess to almighty God, and to you, my brothers and sisters, that I have greatly sinned through my thoughts and in my words, in what I have done, and in what I have failed to do; through my fault, through my fault, through my most grievous fault; therefore I ask blessed Mary ever-Virgin, all the Angels and Saints, and you, my brothers and sisters, to Pray for me to the Lord our God.”
Oh my Jesus, my Savior, I praise You, I bless You, I adore You, I give You thanks for Your great glory. “Lord Jesus Christ, the Only Begotten Son. Lord God, Lamb of God, Son of the Father: You Who take away the sins of the world, have mercy on us.
You Who take away the sins of the world, receive our Prayer. You are seated at the right hand of the Father, have mercy on us. For You alone are the Holy One. You alone are the Lord. You alone are the Most High Jesus Christ; with the Holy Spirit
in the glory of God the Father. Amen.”