In fact the story of General Naaman and the River Jordan is instructive. Naaman was told by Elisha to wash in the river.
Naaman LEFT that instruction because he DID NOT believe.
Only after he'd been more carefully taught did he return and do what he'd been told to do. At that point, there was a modicum of faith.....and he was healed.
If the Christian does not go into the water, those sins are still gone.
He is ofcourse, sinning now as a Christian by disobeying God's commandment, but he is a saved sinner, not a lost one.