John Calvin Commentary Numbers 32:23

"But if ye will not do so, behold, ye have sinned against Jehovah; and be sure your sin will find you out." — Numbers 32:23 (ASV)

But if ye will not do so. He makes a solemn declaration that they will act wickedly if they break their promise, and at the same time proclaims punishment against them, as if he were summoning them before the tribunal of God.

But, although he speaks conditionally of that particular engagement by which the two tribes had voluntarily committed themselves, we may still derive from his words the general doctrine that, unless we keep our promises, God will always be the avenger of fraud and treachery.

The expression, Sin will find you out, is more emphatic than if he had simply said, You shall not escape God’s hand; for its meaning is that vengeance is so connected with sin that it cannot be separated from it.

Thus, in Genesis 4:7, it is said, Sin lieth at the door, to eventually seize the guilty.

For, such is our propensity to sin, that we too often find from experience that we are encouraged to audacity by God’s forbearance, while we think that we have escaped if He acts as if He did not see us for a time.