him who knew no sin

Everyone has sinned.  This is made clear multiple times in the Scripture.  Some of the most well known are Romans 3:23 and Romans 6:23.  
There was only One who didn't sin. Paul makes this point clear in 2 Corinthians:

2Cor. 5:21 He made the one who did not know sin to be sin for us, so that in him we might become the righteousness of God.

He literally stood in place for us so that we could accept His offer of forgiveness and have the assurance of eternal life.  This removes the fear of death and the fear of the future!  Praise God!