Hebrews 3:16-4:2
5 Then he adds, “When it is said, Today if you hear His voice, harden not your hearts, as in the provocation.” This is a transposition, “when it is said, Today if you hear His voice, harden not your hearts.” [It must be read thus:]
“Let us fear lest a promise being left us of entering into His rest, any of you should seem to come short of it; for to us was the Gospel preached as well as unto them when it is said, Today if you hear His voice” (for “Today” is “at every time”).
Then [he adds] “but the word of hearing did not profit them, as they were not mixed by faith with them that heard.” How did it not profit? Then wishing to alarm them, he shows the same thing by what he says:
“For some when they had heard did provoke, howbeit not all that came out of Egypt by Moses: And with whom was He grieved forty years? Was it not with them that had sinned, whose carcasses fell in the wilderness? And to whom swear He that they should not enter into His rest, but to them that believed not? So we see, that they could not enter in because of unbelief.” After again repeating the testimony, he adds also the question, which makes the argument clear. For he said (he repeats), “Today if you hear His voice, harden not your hearts, as in the provocation.” Of whom does he speak (he says) [as] having been hardened? Of whom [as] not believing? Is it not of the Jews?
Now what he says is to this effect. They also heard, as we hear: but no profit came to them. Do not suppose then that by “hearing” what is proclaimed ye will be profited; seeing that they also heard, but derived no benefit because they did not believe.
Caleb then and Joshua, because they agreed not with those who did not believe, escaped the vengeance that was sent forth against them. And see how admirably he said, not, They did not agree, but, “they were not mixed”— that is, they stood apart, but not factiously when all the others had one and the same mind. Here it seems to me that a faction too is hinted at.
Source: Homilies on the Epistle to the Hebrews (New Advent)