Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage. – Galatians 5:1
We are not to stand fast against sin. That is not what this is saying. We are to stand fast in the liberty wherewith Christ has made us free.
We are to stand fast in our position, that we are free, that we are the righteousness of God in Christ (II Corinthians 5:21).
We are not to fight for our victory. God has already given us the victory. We are to fight from our position of already being victorious
We are already victorious because Jesus defeated the devil at the cross.
We have to stand up and act like it.