1 John
King James Version (KJV)

“And every man that hath this hope in him purifieth himself, even as he is pure.”
King James Version (KJV)
3:3 And every one that hath this hope in him - In God.
1Jo 3:3 Every man that hath this hope. To have a hope of being like Christ leads us to try to be like Christ; hence, to be pure "as he is pure".