John Chrysostom

"That being justified by His grace"-again by grace and not by debt-"we may be made heirs according to the hope of eternal life." At the same time there is an incitement to humility, and a hope for the future. For if when we were so abandoned, as to require to be born again, to be saved by grace, to have no good in us, if then He saved us, much more will He save us in the world to come.

Homily on Titus 5 ↗

Theophylact of Ohrid

Again — "by grace," and not by merit. This teaches humility of mind. For we ourselves did not accomplish anything. And it gives encouragement regarding the future. If He saved those who were utterly lost "by grace," how much more will He grant the future to those who have been justified. So that, he says, we might be heirs of eternal life, as we also hope for this. Or: we are already heirs, since we live by hope.

Commentary on Titus ↗

Thomas Aquinas

Then when he says, that, being justified by his grace, he puts down the goal of our salvation, which is the participation of eternal life. Hence he says, heirs. Now, justified is the same as that which he previously called regeneration. In the justification of the unbelieving there are two endpoints, namely, that from which, which is the remission of guilt, and this is renewal; and that toward which, which is the infusion of grace, and this pertains to regeneration. …

Commentary on Titus ↗

Bonaventure

From Christ is the fruit of grace. This fruit of grace is fourfold: it consists in steadying the soul through faith, blessing it through divine love, lifting it up through hope, curbing it through divine fear. The third is mentioned in the Epistle to Titus: "Justified by His grace, we are heirs in the hope of life everlasting."

Collations on the Hexaemeron, Collation 18 ↗