Numbers 32:23 But if ye will not do so, behold, ye have sinned against the LORD: and be sure your sin will find you out.
The two and a half tribes had promised to fight with their brothers till then promised land was divided out. Moses warns them that if they do not keep their word that their sin will be obvious. If they didn’t help their brothers they were sinning against the Lord.
Even hidden sin has a way of coming out. We cannot just sin against God and get away with it. Sin has consequences. They had promised before God and God would be holding them accountable.
We need to understand that all sin is really against the Lord. We might think that it just hurts another person but David in Psalm 51 even made it clear when He said, against thee and thee only have I sinned.
Let’s not take sin lightly. Let’s consider God and pleasing Him instead of ourselves. We can’t play with sin with suffering from the consequences.