"But if you do not do this, you have sinned against the LORD; and be sure your sin will find you out. Build towns for your little ones, and folds for your flocks; but do what you have promised."
The 12 tribes of Israel were about to embark on a long military campaign as they settled into the "Promised Land" they believed was given them by God. All were expected to participate. Land would be divided equitably afterwards.
The Reubenites & Gadites were cattle ranchers. They surveyed the land of Gilead as prime cattle-raising country. The only problem was that Gilead wasn't int he official "Promised Land" area of Canaan they were going to be living. So they asked Moses if they could remain on this side of the Jordan River in Gilead. Simple request, right? Not like you'd think...
Moses, being the "brains of the unit" could immediately see a problem. Anytime one moves into a new place, one's immediate foreseeable future will be bound up with "settling in." Moses was afraid they'd be too busy to help the rest of the tribes int eh upcoming battle(s). So he made a deal: they could take the land of Gilead (and get their flocks & family "comfy"), but they themselves couldn't settle in until AFTER they fought with their brothers in battle. The tribes of Reuben & Gad wholeheartedly agreed.
Moses then set before them an oath, which they willingly took. And he said he'd hold them to their word. Then he concluded with this caveat: "But if you do not do this, you have sinned against the LORD; and be sure your sin will find you out" (v.23).
How true. Our sins will find us out. Maybe not immediately (though it often happens that way)... maybe not in this lifetime (though often it happens this way)... but ultimately, our sins will find us out. And as Moses implied, all sin is sin against God. We may think it's only against another, but it's so much more.
So be forewarned...
But thanks be to God that we have a remedy for our sin... Grace, forgiveness, & reconciliation is available 100% of the time - if we seek it! Jesus stands ready to forgive us when our sins find us out. Over, and over, and over, and over...