13 There remains still the question, which I now think may meanwhile thus be solved. There is a voice of some kind—there is, I say, a certain kind of voice of the Shepherd, in respect of which the sheep hear not strangers, and in respect of which those who are not sheep do not hear Christ. What a word is this! “He that endures to the end, the same shall be saved.” No one of His own is indifferent to such a voice, a stranger does not hear it: for this reason also does He announce it to the former, that he may abide perseveringly with Himself to the end; but by one who is wanting in such persevering continuance with Him, such a word remains unheard.
One has come to Christ, and has heard word after word of one kind and another, all of them true, all of them salutary; and among all the rest is also this utterance, “He that endures to the end, the same shall be saved.” He who has heard this is one of the sheep. But there was, perhaps, some one listening to it, who treated it with dislike, with coldness, and heard it as that of a stranger. If he was predestinated, he strayed for the time, but he was not lost for ever: he returns to hear what he has neglected, to do what he has heard.
For if he is one of those who are predestinated, then both his very wandering and his future conversion have been foreknown by God: if he has strayed away, he will return to hear that voice of the Shepherd, and to follow Him who says, “He that endures to the end, the same shall be saved.” A good voice, brethren, it is; true and shepherd-like, the very voice of salvation in the tabernacles of the righteous. For it is easy to hear Christ, easy to praise the gospel, easy to applaud the preacher: but to endure unto the end, is peculiar to the sheep who hear the Shepherd's voice.
A temptation befalls you, endure thou to the end, for the temptation will not endure to the end. And what is that end to which you shall endure? Even till you reach the end of your pathway. For as long as you hear not Christ, He is your adversary in the pathway, that is, in this mortal life. And what does He say? “Agree with your adversary quickly, while you are in the way with him.” You have heard, hast believed, hast agreed. If you have been at enmity, agree. If you have got the opportunity of coming to an agreement, keep not up the quarrel longer.
For you know not when your way will be ended, and it is known to Him. If you are a sheep, and if you endure to the end, you shall be saved: and therefore it is that His own despise not that voice, and strangers hear it not. According to my ability, as He gave me the power, I have either explained to you or gone over with you a subject of great profundity. If any have failed fully to understand, let him retain his piety, and the truth will be revealed: and let not those who have understood vaunt themselves as swifter at the expense of the slower, lest in their vaunting they turn out of the track, and the slower more easily attain the goal. But let all of us be guided by Him to whom we say, “Lead me, O Lord, in Your way, and I will walk in Your truth.”
Source: Tractates on the Gospel of John (New Advent)