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.

Moses said unto them, If ye will do this thing--with sincerity and zeal.

go before the Lord to war--The phrase was used in allusion to the order of march in which the tribes of Reuben and Gad immediately preceded the ark (see on Num. 2:10-31), or to the passage over the Jordan, in which the ark stood in mid-channel, while all the tribes marched by in succession (Jos 3:4), of course including those of Reuben and Gad, so that, literally, they passed over before the Lord and before the rest of Israel (Jos 4:13). Perhaps, however, the phrase is used merely in a general sense to denote their marching on an expedition, the purpose of which was blessed with the presence, and destined to promote the glory, of God. The displeasure which Moses had felt on the first mention of their proposal had disappeared on the strength of their solemn assurances. But a lurking suspicion of their motives seems still to have been lingering in his mind--he continued to speak to them in an admonitory strain; and he concluded by warning them that in case of their failing to redeem their pledge, the judgments of an offended God would assuredly fall upon them. This emphatic caution against such an eventuality throws a strong doubt on the honesty of their first intentions; and yet, whether through the opposing attitude or the strong invectives of Moses they had been brought to a better state of mind, their final reply showed that now all was right.