But if you do not what you say, no man can doubt but you sin against God: and know ye, that your sin shall overtake you.

'And if ye do not so, lo, ye have sinned against Jehovah, and know ye your sin, that it doth find you;

But if you do not do this, then you are sinners against the Lord; and you may be certain that your sin will have its reward.

But if you do not do what you have said, no one could doubt that you will have sinned against God. And know this: your sin shall overtake you.

Quod si non feceritis ita, ecce, peccastis Jehovae, et perpendite peccatum vestrum quod invenient vos.

But if ye will not do so. He makes a solemn protestation that they will deal wickedly, if they break their promise: and at the same time denounces punishment against them, as if he were summoning them before the tribunal of God. But, although he speaks conditionally of that particular engagement, whereby the two tribes had voluntarily bound themselves, still we may derive from his words the general doctrine, that, unless we abide by our promises, God will always be the avenger of fraud and treachery. The expression, "Sin will find you out," is more emphatic than as if he had simply said, You shall not escape God's hand; for the meaning of it is that vengeance is so connected with sin, that it cannot be severed from it. Thus, in Genesis 4:7, it is said, "Sin lieth at the door," to lay hold at length of 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, whilst we think that we have escaped, if He makes as though He saw us not for a time.

Be sure your sin will find you out - literally, "know ye your sin that it will find you out." Moses implies that their sin would eventually bring its own punishment along with it.

But if ye will not do so, behold, ye have sinned against the LORD: and be sure your sin (i) will find you out.
(i) You shall assuredly be punished for your sin.

But if ye will not do so,.... As they promised they would, and Moses insisted on it that they should:
behold, ye have sinned against the Lord making such a request, and not fulfilling the conditions on which it was granted:
and be sure your sin will find you out; fly in their faces, accuse them in their consciences, charge and load them with guilt, and bring deserved punishment upon them: sin may be put, as it often is, for the punishment of sin, which sooner or later will find out and come upon the impenitent and unpardoned sinner.

Your sin - The punishment of your sin. Sin will certainly find out the sinner sooner or later. It concerns us therefore to find our sins out, that we may repent of them and forsake them, lest our sins find us out, to our confusion and destruction.