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)