Let us not suppose, then, that the Holy Spirit, by this promise [of eternal life], commends the dignity of our works, as if they were deserving of such a reward. For Scripture leaves us nothing of which we may glory in the sight of God. Nay, rather its whole object is to repress, humble, cast down, and completely crush our pride. But in this way help is given to our weakness, which would immediately give way were it not sustained by this expectation, and soothed by this comfort.
First, let every man reflect for himself how hard it is not only to leave all things, but to leave and abjure one’s self. And yet this is the training by which Christ initiates his disciples, that is, all the godly.
Secondly, he thus keeps them all their lifetime under the discipline of the cross, lest they should allow their heart to long for or confide in present good.
In short, his treatment is usually such, that wherever they turn their eyes, as far as this world extends, they see nothing before them but despair; and hence Paul says “If in this life only we have hope in Christ, we are of all men most miserable,” (1 Cor. 15:19). That they may not fail in these great straits, the Lord is present reminding them to lift their head higher and extend their view farther, that in him they may find a happiness which they see not in the world.
John Calvin, Institutes, III.18.4