Inasmuch, then, as the true believers are also not just simply and primarily here to see a reward in this life, but rather for the promise of eternal life and along with that, are to commend to God the Lord how He may desire to deal with them in this life, whether He wants to lead them to heaven via roses or via thorns, whether He wants to give them cross and adversity or relief from tribulation… all this they are to willingly receive from His hand and in all things, obediently commend themselves into His gracious, fatherly hand.

This is also the Scriptural explanation given to Timothy by this apostolic rule: Train yourself in godliness, that is serium pietatis studium [the serious pursuit of godliness]. For bodily training is of little value, but godliness is good for everything and has the promise of this and the future life, that is amplissimum pietatis proemium [the farthest reaching reward for godliness].

By relying on divine help, let us and others, by responsibly following this apostolic rule, train ourselves for godliness. [Let us do it] in such a manner that we, at the beginning, search out the most important and moving reasons which should, in general, encourage and awaken each individual to godliness, which yet not everyone makes their top priority. Let us sharpen these skills. (Schola Pietatus-Volume 1 by Johann Gerhard, Repristination Press, p. 28)

Second, as we consider this benefit of godliness is the matter that by the promise of this life is understood to mean the reward in this life cum exceptione crucis & salutis [with the exception of the cross and salvation]. With this is indicated that it will not redound to our salvation and be of benefit to us that God the Lord, in this life, lays upon us the fatherly chastisement of crosses. Christ teaches us this in Mark 10:29–30: Anyone who forsakes house or brother or sister or father or mother or wife or children or land for My sake, and for the sake of the Gospel, will in this present time, receive a hundred-fold of houses and brothers and sisters and mothers and children and land amidst persecution; and in the future world, eternal life. Here, we clearly hear that the promise of this life or the reward of godliness in this life, takes place amidst persecution; that is to say, this will not occur apart from cross and adversity, for all who desire to live godly in Christ Jesus will have to suffer persecution (2 Tim. 3:12). My child, if you wish to be a servant of God, then prepare yourself for anxiety (Sir. 2:1). Since you love God, it has to be that you will not remain without anxiety, in order that you be preserved (Tob. 12:13).
It would be very easy for God to richly grant the godly everything good in this life; for Him to give them relief from all anxiety, tribulation and adversity; along with this, to give them riches, honor, health and all blessings. However, He knows very well that the godly still bear flesh and blood within them, which so very easily could abuse all of this and bring upon itself eternal retaliation. That’s why He keeps them under the cross in this life and, at the same time, He does good things for them in the midst of cross and tribulation, in that, through the Holy Spirit, He mightily comforts and strengthens them, assures them of His grace, makes them to be happy and confident that they with David may say, from Psalm 119:71: I love that You have humbled me—understand, by means of cross and adversity—so that I may learn of Your righteousness. (Schola Pietatus-Volume 1 by Johann Gerhard, Repristination Press, p. 26-27)

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)