For he well knows how we are all generally minded: that we first look for the benefit, and soon we take care to ask: Cui bono? (Who benefits from this?) How am I improved by this? What may I expect from this if I become godly? Just as with the Israelites, this kind of question and complaint was raised in Mal. 3:14—It is in vain that one serves God; and what does it benefit us that we keep His command and lead a difficult life for the Lord of Sabaoth? However, here God the Lord Himself answers: it shall not be in vain, and He will in no way forget what one had done in service to Him. The God-fearing, He says in v. 15, comfort each other in this way: The Lord takes note and hears and a reminder is written for Him about those who fear the Lord and remember His Name (v. 17). They shall, says the Lord of Sabaoth, be Mine inheritance in the day which I shall create, that is, they shall be My own special people who will be loved by Me like a precious treasure (Exo. 19:5, Psa. 135:4, Tit. 2:4) and I will take care of them like a man takes care of his son who serves him. That is to say, when I punish the godless, I will protect the godly and perform goodness for them (v. 8). And you will once again see what a difference there is between one who serves God and one who does not serve Him. For the former shall receive protection and rescue from tribulation, along with prosperity and blessings. The latter, however, will receive punishment and misfortune for this, even also in this life.
In the same manner, the holy apostle especially states here: godliness is not without its outstandingly great benefits. Instead, it contains the promise of this life and the future life. In other words, God the Lord has made the sure, unfailing promise that for those who zealously pursue godliness, He will do good for them here in time and there in eternity. It will fare well for them in this life, and in everlasting life He will pour out on them heavenly blessings (Heb 6:10). God is not unrighteous so that He will forget your works and labor of love which you have shown in His name, as you served the saints and still serve them. That is, it will conflict with the righteousness of God if He does not richly reward with all sorts of blessings those who do good and practice godliness. The holy apostle explains such a glorious benefit from godliness per antithesin, sive collationem imparium [by antithesis or comparison of any quality].
(Schola Pietatus-Volume 1 by Johann Gerhard, Repristination Press, p. 20-22)