Hebrews 4:6

We also had the promise to enter into the kingdom through our faith and spiritual way of life, as well as those who accepted the command through the law so that … they might possess the land granted to them. “But the message” of the law “which they heard did not benefit them because it did not meet with faith” in the hearers. “We who have believed” in Christ and his gifts “enter” faithfully “that rest.” They, on the other hand, did not enter into that rest in consequence of the vow made through David, who said, “I swore in my anger that they should not enter my rest.”

In fact, if Joshua, the son of Nun, who allowed them to inherit the land, had settled them and given them rest, they still would not speak at all about the “other day of rest.” Indeed, Joshua made them rest, because he gave them the land as an inheritance, but they did not rest in it perfectly, as God perfectly rested from God’s works, for they lived in toils and wars. If that rest was not a true rest, since Joshua himself, the giver of their rest, was urged by the wars, if this is their condition, I say, there still remains the sabbath of God, who gives rest to those who enter there, as God rested from God’s works, that is, from all the works which God made.

"Seeing therefore it remaineth" (he says) "that some must enter therein, and they to whom it was first preached entered not in because of unbelief: again he limiteth a certain day, saying in David, To-day, after so long a time; as it has been said before." But what is it that he means? "Seeing then" (he means) that "some must" certainly "enter in," and "they did not enter in." And that an entrance is proclaimed, and that "some must enter in," let us hear from what this is clear. Because after so many years (he says) David again says: "To-day if ye will hear His voice, harden not your hearts."

It was fitting for them to be afraid, he says so that they might not be found lacking because of the depravity of their opinion, when they themselves had received the promise of access into the rest. For let no one suppose that the promise of the things to come are sufficient for him, just as it was not sufficient for them. For they were not joined to the things promised in accordance with faith. Therefore, one ought to read as follows, “They did not attach themselves in faith to the things that they heard,” namely, the promises that were made to them from God through Moses.

We ought to be afraid … lest we, too, who have received the promise of the entrance into the rest, may be found lacking through a wickedness of purpose. For let no one think that the promise of things to come suffices him any more than it did them. Indeed, they did not understand in faith what had been promised.

This is the work of true “rest,” namely, not having to run again to the old things, while enduring transition and change. For just as God is said to rest from his creation of the world, having completed its foundation, so it is fitting that also the one who has entered “into rest” not run back again to the old things, viewing with contempt the labors required by the law’s virtuous ordinances to restrain transgression. For out of necessity change and a removal from the old institution follows these things.

“Today” is neither an indefinite period of time, nor can it be predicated of an interval outside of “days” as if “today” were joined with each day. This should be stated not only for the sake of apostolic perspicuity but also for the sake of those who wish to understand, in the matter of the origin of the Only Begotten, the “I have begotten you today” by flatly asserting that the “today” is an indefinite period of time. They cannot perceive that if this were the case one would not be able to speak of a “today” since there was then not yet a day. The apostle has made clear that he would not say that “today” is an indefinite period of time when he said, “Again speaking in David he marks off a day.” Also in another passage, “Comfort them every day until it will be called ‘the Day.’ ” First he showed that the “today” is not being spoken of outside of “days,” by his saying “every day,” and so appending “until it will be called ‘the Day.’ ” Then in the matter of the “day” it does not appear as if he were talking about an indefinite period of time which might be applied both to the time that has already passed and the time that is about to come. For what does he say? Deeming “each day” to be “the Day” which is spoken to we should give heed to remaining in the faith. For as “the Day” indicates the present day, he advises them to make full use of the day for that which is useful for exhortation.

After making this digression to scare them and to cause them to look forward to the hope given them, he then gives attention to the “rest,” bringing out that in times past the inspired David foretold it to us.

Hearing the words does not suffice for salvation; accepting it in faith is necessary, and holding it firm. After all, what benefit was God’s promise to those who received it, but did not receive it faithfully, trust in the power of God or, as it were, associate closely with God’s words?

As the God of all on the sixth day completed the whole of creation, and on the seventh he rested from creating, so those departing this life and moving to that one will be rid of the present labors.

He wants to make clear three rests mentioned in the divine Scripture: first, the seventh day, on which God finished creating; second, the land of promise; and third, the kingdom of heaven. He provides proof of this from the inspired testimony: If there is no other rest (he is saying), why on earth does he also urge those in receipt of the second kind not to harden their hearts, threaten punishment and make mention of those who spurned the second kind? He cites them in order, and firstly the rest on the seventh day.

I am speaking of that order of tears which belongs to those who shed tears unceasingly both night and day. Whoever has found the reality of these things truly and accurately has found it in stillness. The eyes of such a man become like fountains of water for two years’ time or even more, that is, during the time of transition: I mean, of mystical transition. But afterwards you enter into peace of thought; and from this peace of thought you enter into the rest of which St. Paul has spoken, but only in part and to the extent that nature can contain it. From that peaceful rest his intellect begins to behold mysteries. And thereupon the Holy Spirit begins to reveal heavenly things to you, and God dwells within him and raises up the fruit of the Spirit in you. And from this he perceive dimly … the change nature is going to undergo at the renewal of all things.… When you enter into that region which is peace of the thought, then the multitude of tears is taken away from you, and afterwards tears come to you in due measure and at the appropriate time. This is, in all exactness, the truth of the matter as told in brief, and it is believed by the whole church.

Just as the first “rest” did not prevent there being a second rest, so neither does the existence of a second rest prevent the existence of a third and more perfect rest.… Then it is clear that there is a certain other rest beyond those rests which have been spoken of, and that this rest is hallowed not for any who happen to chance upon it, but rather “for the people of God.” But truly the people of God are “those who believe” in him and who keep his commandments.

He says, “Not having joined themselves to the things they heard,” that is, the things they believed. How was it possible for them to be joined to those things? “By faith,” he says, that is, through faith. For if the latter had believed as the former had, they would have been joined together into one, since their faith would have joined them together and blended them together.

In another way. “rested from his works, as God did from his own.” As if, he rested. Since he has demonstrated that there is a third rest, and that some will enter into it, he comes to the conclusion and says: "From what has been said, it is certainly evident that there is another rest besides the aforementioned, and this is not for everyone, but dedicated to the people of God." Truly, the people of God are those who have believed in Him and keep His commandments.

"for the people of God." He says to the faithful. And how is this evident? From the fact that David commands, saying: Do not harden your hearts. For he would not command this if there were no sabbath and rest. (What benefit would there be for those who obeyed?) Nor would it be commanded to us, not to do the same as the Jews who were in the desert, lest we too be deprived of rest in the same way.
"who has entered."Since they were weary from evils, there was no mention made of the countless goods that were to come, he comforts them by saying that he who has entered into the third rest, which is the rest of the kingdom of heaven, rests from his works, that is, from persecutions, temptations, afflictions, just as indeed God rested from the works He had done in the first rest.

“Therefore, a Sabbath-keepingremains.” Sabbath-keeping or rest. For sabbaths are interpreted as rest. Therefore, a certain third rest remains, which is the kingdom of heaven. However, he called it Sabbath-keeping, from the primary rest which was of the Sabbath-keeping.

"And he himself rested." As if he had rested.

For if some must enter, and those who entered have not entered into the rest of the promised land, it again defines a third rest, so that, as he says, some may enter into it. For if God did not have a third rest, He would not have said: "Today, if you hear His voice, do not harden your hearts," so that there may be a rest into which those should enter who do not harden their hearts. Since obedience would be completely useless if there were neither rest nor reward. You have seen how he confirms it? "Do not harden your hearts so that you may enter into the rest," he says. For by saying that those who hardened their hearts did not enter into the rest, he implies that those who do not harden their hearts will enter. For every command of God has also a future reckoning.
"For if Joshua had given them rest." For if that were the only rest, namely the promised land into which they entered under the leadership of Joshua, he would not have spoken of another day of rest through David. But therefore, he mentions it, saying: Today, and he gave a command, saying: Do not harden your hearts: as if there exists a third rest into which it is necessary for those who are obedient to enter.