Calvin's Commentaries, Vol. 5: Harmony of the Law, Part III, tr. by John King, [1847-50], at sacred-texts.com
28. Observe and hear all these words which I command thee, that it may go well with thee, and with thy children after thee for ever, when thou does that which is good and right in the sight of the Lord thy God.
28. Custodi, et audi omnia verba ista quae ego praecipio tibi, ut bene sit tibi et fillis tuis post te usque in saeculum, quum feceris quod bonum est et rectum in oculis Jehovae Dei tui.
Here, again, God invites the obedience of the people by the promise of reward; not that the hope of reward at all avails in itself to arouse men, but because He would thus keep all under the conviction of their just condemnation: for how will it help them to answer that they are not sufficient to perform what God requires, when it appears that they are thus wretched through their own fault? But, as has been said before, it is profitable by indulgence to believers that the reward of obedience should be promised them when they have kept the Law, since their innumerable defects are not imputed to them. Still this doctrine remains sure, that if men devote themselves to the keeping of the Law, God, although He owes them nothing, will nevertheless faithfully reward them.