Paul offered from the book of Romans:

Romans 3:23 - For all have sinned and fall short of the glory of God,

This was an indictment on the whole of mankind, from Adam forward. We all do and say and even think things that are wrong, regardless of whether we are Christians or not.

But the greatest error or sin each of us can make in this life is to reject Jesus Christ as our Lord and Savior.

John 14:6 - Jesus said to him, “I am the way, and the truth, and the life. No one comes to the Father except through me.

By rejecting Christ and the sacrifice He made for us on the cross, we are rejecting the Father (God) as well. And to reject God is to place ourselves in eternal jeopardy of permanent separation from Him in this life and on the other side of the grave.

2 Corinthians 5:10 - For we must all appear before the judgment seat of Christ, so that each one may receive what is due for what he has done in the body, whether good or evil.

However, the provision the Lord has made for His children is to escape the payment of our sins. And the outpouring of our love and worship of God becomes evident in a changed life that includes the love of our neighbors.

Matthew 25:31-41 - “When the Son of Man comes in his glory, and all the angels with him, then he will sit on his glorious throne. Before him will be gathered all the nations, and he will separate people one from another as a shepherd separates the sheep from the goats. And he will place the sheep on his right, but the goats on the left. Then the King will say to those on his right, ‘Come, you who are blessed by my Father, inherit the kingdom prepared for you from the foundation of the world... Then he will say to those on his left, ‘Depart from me, you cursed, into the eternal fire prepared for the devil and his angels."

But there is an order we cannot skip over. It is to love God as the first commandment and then, from that position, love our neighbor.

Matthew 22:37-40 - You shall love the Lord your God with all your heart and with all your soul and with all your mind. This is the great and first commandment. And a second is like it: You shall love your neighbor as yourself. On these two commandments depend all the Law and the Prophets.”

And the greatest expression of that love of God is to read and follow His Word, the Bible, His love letter to mankind. The greatest sin we can commit is to reject that love and submission to the Creator of the universe.