How are we to understand this promise?

The Word of God compares the heart of man to an eye and speaks of mature Christians as having “the eyes of your heart enlightened” (Eph 1:18). As the clear eye can see the light, so the pure heart can behold God. Since the vision of God’s countenance is the very source of everlasting blessedness, the promise of seeing God is the promise of blessedness in its highest degree.

This purification is the work of grace and of ascetic vigilance together: “Create in me a clean heart, O God, and renew a right spirit within me” (Psa 51:10 [50 LXX]; see Q307). The vision itself transforms: “beholding as in a mirror the glory of the Lord, we are all being transformed into the same image, from glory to glory” (2Co 3:18; see Q011, Q383), until that day when “we shall see him as he is” and be made like him (1Jo 3:2–3: “and everyone who has this hope in him purifies himself, as he is pure”; see Q254, Q383).