Numbers 32:23, 25 But if ye will not do so, ye have sinned against the Lord: and be sure your sin will find you out. And the children of Gad and the children of Reuben spake unto Moses, saying, Thy servants will do as my lord commanded.
The 2 1/2 tribes settling east of Jordan Promised to fight with Israel in the west. Moses held them to their promise, and warned them that if they did not keep their word it would be a sin against God. And he further warned: “your sin will find you out.” They responded: “we will do.”
Breaking a promise is similar to lying; one didn’t really mean it. Moses called it a sin and warned that it will be discovered, because sin always has consequences. Bad things result that expose the sin and convict the sinner. The ultimate end will be that Jesus died for the sinner and forgives the sin; we believe the gospel and have life.
Meanwhile, we live in a sinful world, suffering the wages of sin. Faithfulness and trustworthiness are noble and desirable qualities that the Lord works in the believer through the Word. Life runs a lot better when people do what they say, keep promises, and fulfill commitments. But since sinners don’t, society must make laws to enforce commitment. The Christian does not need to be forced, for he responds faithfully to a faithful God. When we fail, we trust in the faithfulness of God, Jesus did it for us.
The rest of the story is that the 2 1/2 tribes did do as they promised.