John Chrysostom
He says that there are three kinds of rest. One is the Sabbath rest, in which God rested from His works.
Another is the Palestinian rest, when the Jews, having entered Palestine, were to rest from many calamities and labors.
The third and true rest is the Kingdom of Heaven. Whoever reaches it truly rests from labors and sorrows. Thus he mentions three kinds of rest here.
Why, then, when discussing one rest, did he mention three? To show that the prophet spoke of the last.
He did not speak of the first, because it had existed long before. Nor did he speak of the second, which was in Palestine, because that too had already ended.
Rather, he says, "They shall not enter My rest." The third remains, and it is this of which he speaks.
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If David later, after that generation, said, "Today, if you hear His voice, do not harden your hearts, as in the time of murmuring," why did he say it?
It was so that you would not suffer what your forefathers suffered, and would not be deprived of rest. Thus it is clear that in his words he pointed to another rest.
If he were speaking of the rest they had already attained, why would he have said to them again: "Today, if you hear His voice, do not harden your hearts, as in the time of murmuring"?
What other rest is this, except the Kingdom of Heaven, of which the Sabbath is an image and likeness?
Having set forth the whole testimony, which is as follows, "Today, if you hear His voice, do not harden your hearts, as in the time of murmuring, in the day of testing in the wilderness, where your fathers tested Me, proved Me, and saw My works for forty years.
Therefore I was indignant with that generation and said: They continually go astray in heart; they have not known My ways. Therefore I swore in My wrath that they shall not enter My rest," the Apostle then says: "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God."