“Let not sin therefore reign in your mortal body, to make you obey its passions.” (Rom. 6:12)
The difference between sin reigning in a Christian and sin merely remaining in a Christian is the active ongoing resistance of a Christian. As Thomas Manton has said, “Sin reigneth where it is not resisted; it only remaineth in you, where it is opposed.”
– Thomas Manton, An Exposition of the Epistle of James, p. 267.