“Sin is knowingly choosing to do wrong or not to do right” (Plan of Salvation, p. 9, emphasis added. Plan of Salvation is a pamphlet often shared by LDS missionaries). Defining sin as something we knowingly do is one of the ways Mormonism waters down sin.
The Bible says differently. Most Mormons are unaware of and have not considered this, so it can be a good topic of discussion.
A good passage to share, because it will be unfamiliar to almost every Mormon, is Leviticus 5:17–18.
And if a soul sin, and commit any of these things which are forbidden to be done by the commandments of the Lord; though he wist it not, yet is he guilty, and shall bear his iniquity. And he shall bring a ram without blemish out of the flock, with thy estimation, for a trespass offering, unto the priest: and the priest shall make an atonement for him concerning his ignorance wherein he erred and wist it not, and it shall be forgiven him.
Leviticus 5:17–18
Even though this verse is part of the Old Testament sacrificial law, it still makes the point that we are guilty even when we sin and don’t know it. “Wist” is an old English word for “know.” Note how:
- The person’s action is labeled a sin
- It makes him guilty
- It needs atonement
- So he can be forgiven
Emphasize each point. Highlight how not knowing he sinned but still being guilty is mentioned twice in the passage. This passage clearly shows that sin is not limited to “knowingly choosing” to do a sinful action.
(The LDS edition of the KJV references the subject of “accountability” in relation to this passage. There is a good chance they will see this, and it will support the point you make.)
Another passage you could use is Jesus’ prayer from the cross:
Then said Jesus, Father, forgive them; for they know not what they do.
Luke 23:34
Ask why Jesus prayed this if sin is only something people “knowingly choose” to do.
Your goal in sharing these passages is to get them to think. The more we can show them that sin is much more common than Mormonism says, the more they will see their inability to keep the commandments and their great need for a Savior who kept them for them.