7 “And every one that has this hope in Him.” You see how he has set us our place, in “hope.” You see how the Apostle Paul agrees with his fellow-apostle, “By hope we are saved. But hope that is seen, is not hope: for what a man sees, why does he hope for? For if what we see not, we hope for, by patience we wait for it.” This very patience exercises desire. Continue, for He continues: and persevere in walking, that you may reach the goal: for that to which you tend will not remove.
See: “And every one that has this hope in Him, purifies himself even as He is pure.” See how he has not taken away free-will, in that he says, “purifies himself.” Who purifies us but God? Yea, but God does not purify you if you be unwilling. Therefore, in that you join your will to God, in that you purify yourself. Thou purifiest yourself, not by yourself, but by Him who comes to inhabit you. Still, because you do somewhat therein by the will, therefore is somewhat attributed to you. But it is attributed to you only to the end you should say, as in the Psalm, “Be my helper, forsake me not.” If you say, “Be my helper,” you do somewhat: for if you be doing nothing, how should He be said to “help” you?
Source: Homilies on the First Epistle of John (New Advent)