Assembly is the proper translation of the word "church." The righteous are those who are "right with God." After the resurrection people are made right with God by trusting Jesus. Those who trust Jesus are born of the spirit (John 3:6-8), also called born again, born from above, or born of God. They have a new living spirit inside of them that is born of God. It is righteous, holy, and blameless just as Jesus is.
God has given us this perfect spirit as our new identity. God does not identify us as sinners anymore. He took our sins away on the cross and remembers them no longer.
We are the assembly, the body of Jesus and no member of our Lord's body is sinful. We are the spiritual body of our risen Lord.
"Sinners do not stand in the assembly of the righteous because, thanks to our Messiah Jesus, the righteous are not sinners but children of God."