If, then, the promise of the land of Canaan is a pure promise of grace, how much more will not the promise of eternal life be a pure promise of grace, which flows forth simply and solely out of the undeserved grace of God, and in no way depends on the merits of one’s own works? When you have done everything that I have commanded, then still say, we are non-beneficial servants; we have done what we were obligated to do. That’s how Jesus teaches us in Luke 17:10: With what would we be able to earn the promise of eternal life? If someone does a particular thing, but does not actually perfectly fulfill what he already owes, he cannot earn anything with his works.
Now, however, we humans have this privilege: that all the things we do to glorify God and do to please Him, we are to do since we already owe Him that jure creationis, conservationis & redemptionis [by right of creation, preservation and redemption] because He has created, preserved, and redeemed us; indeed, we do not do this perfectly on account of the fact that in this life, we do not perfectly do all these things; how, then, would we be able to earn anything from God the Lord with our indebted imperfection? Oh! The poor cannot earn it! Yet we as poor beggars have to petition and beg before God’s door for daily bread (Mat. 6:11). How dare we ever more deeply fall into this line of thinking that we might be able earn heaven and eternal life from God the Lord by our good works? The holy apostle says it well in Rom. 8:18—I maintain that the sufferings in these times is not worth compare to the glory which shall be revealed to us. If the sufferings of this present time are not worth comparing to eternal glory, then suffering for the sake of God’s glory is of much weightier and greater importance than works. And in Rom. 6:23: Eternal life is (not a payment or an earned wage from good works, but a gift of God χάρισμα, a gift of grace) in Christ Jesus our Lord, which He, through His precious merit and perfect obedience, won for us. … What God the Lord has promised us, He gives to us on the grounds of grace, not as a reward for our works. (Schola Pietatus-Volume 1 by Johann Gerhard, Repristination Press, p. 25-26)

This is also the second part of this apostolic verse; namely, the important reason which should motivate us so that with the utmost zeal, we should train in godliness. This, then, is the greatest overwhelming benefit which we shall derive from this, because godliness is good for all things and holds the promise for this and the future life, for which bodily training is of little value.
With this, however, of necessity we are to take note of two things: First of all, that such a rich bestowal of godliness—as one experiences it in this and also in the future life—takes place by sheer grace and not by our earning it by works. The apostle teaches this when he says: Godliness possesses the promise of this and the future life. For if it is a promise, then it cannot be something earned, especially since the promise flows forth from sheer unearned grace. Concerning the promise of the land of Canaan given to the Israelites, God the Lord says the following through Moses to His people in Deu. 9:4-5: Do not say in your heart: The Lord has led me in to take this land because of my righteousness, for you are not entering in to receive this land of the Gentiles because of your righteousness and because of the uprightness of your heart. And later He repeats this again in v. 6—So then, realize that the heart of your God does not give you this good land to enter into because of your righteousness, especially since you are a stiff-necked people. (Schola Pietatus-Volume 1 by Johann Gerhard, Repristination Press, p. 24-25)

For bodily training by which we discipline the body in food and drink and refrain from certain foods (1 Cor. 9:25), as well as chastising the body by fasting, these practices indeed have their good benefits so that one might be able to more actively meet the obligation for prayer and godliness. Yet this still does not instill true holiness; does not cleanse the heart of sin; nor does it, in and of itself, make us acceptable to God. Thus, these are of little value if we practice and uphold them in opposition to true inward godliness which consists of true inward heart-felt repentance, true humble faith in Christ, true heart-felt love in the fear of God and other holy, inward motivations of the heart. For this, godliness has the promise of this and the future life. Accordingly, what the holy apostle specifically teaches here about bodily training being of little value is one and the same view and meaning that he teaches in other places about such outward practices and distinctions about food, Rom. 14:3: Whoever eats should not despise him who does not eat; and whoever does not eat, let him not judge the one who eats, for God has accepted him. V. 6: Whoever eats, let him eat for the Lord, for he thanks God. V. 17-18: For the Kingdom of God is not eating and drinking, but rather righteousness and peace and joy in the Holy Spirit. Whoever serves God this way is pleasing to God and acceptable to mankind. 1 Cor. 8:8—Food does not draw us to God. If we eat, we will not thereby be any better. If we do not eat, we will not on that account be any less. Heb. 13:9—It is a precious matter that the heart become steadfast. This occurs by grace, not by food. None benefit who circumvent this, that is, those who want to serve God in this way. For it can only have a benefit, as the apostle testifies in this verse, if one uses this as an external discipline and Paedagogiam [pedagogy], without any delusion of necessity, as a special service of worship. (Schola Pietatus-Volume 1 by Johann Gerhard, Repristination Press, p. 23-24)