But there is some more difficulty in what he immediately subjoins, that there is another today appointed for us in the Psalm, because the former people had been excluded; but the words of David (as it may be said) seem to express no such thing, and mean only this, that God punished the unbelief of the people by refusing to them the possession of the land. To this I answer, that the inference is correct, that to us is offered what was denied to them; for the Holy Spirit reminds and warns us, that we may not do the same thing so as to incur the same punishment. For how does the matter stand? Were nothing at this day promised, how could this warning be suitable, "Take heed lest the same thing happen to you as to the fathers." Rightly then does the Apostle say, that as the fathers' unbelief deprived them of the promised possession, the promise is renewed to their children, so that they may possess what had been neglected by their fathers.

Again, he limiteth - He designates, or definitely mentions. The word rendered "limiteth" - ὁρίζει horizei - means to "bound," to set a boundary - as of a field or farm; and then to determine or fix definitely, to designate, appoint. Here it means, that he specifies particularly, or mentions expressly.
A certain day - A particular time; he mentions today particularly. That is, in the time of David, he uses the word "today," as if time was "then" an offer of rest, and as if it were then possible to enter into it. The object of the additional thought was to show that the offer of rest was not confined to the Israelites to whom it was first made; that David regarded it as existing in his day; and that man might even then be invited to come and partake of the rest that was promised. "Nearly five hundred years after the time when the Israelites were going to the promised land, and when the offer of rest was made to them, we hear David speaking of "rest" still; rest which Was offered in his time, and which might then be lost by hardening the heart. It could not be, therefore, that the offer of rest pertained merely to the promised land. It must be something in advance of that. It must be something existing in the time of David. It must be an offer of heaven." A Jew might feel the force of this argument more than we do; still it is conclusive to prove the point under consideration, that there was a rest spoken of long after the offer of the promised land, and that all the promises could not have pertained to that.
Saying in David - In a Psalm composed by David, or rather perhaps, saying "by" David; that is, God spake by him.
Today - Now - that is, even in the time of David.
After so long a time - That is, so long after the first promise was made; to wit, about 500 years. These are the words of Paul calling attention to the fact that so long a time after the entrance into the promised land there was still a speaking of "today," as if even then they were called to partake of the rest.
As it is said - To quote it exactly; or to bring the express authority of the Scriptures. It is expressly said even after that long time, "today - or now, if you will hear his voice." All this is to prove that even in that time there was an offer of rest.

He limiteth a certain day - The term day signifies not only time in general, but also present time, and a particular space. Day here seems to have the same meaning as rest in some other parts of this verse. The day or time of rest relative to the ancient Jews being over and past, and a long time having elapsed between God's displeasure shown to the disobedient Jews in the wilderness and the days of David, and the true rest not having been enjoyed, God in his mercy has instituted another day - has given another dispensation of mercy and goodness by Christ Jesus; and now it may be said, as formerly, To-day, if ye will hear his voice, harden not your hearts. God speaks now as he spoke before; his voice is in the Gospel as it was in the law. Believe, love, obey, and ye shall enter into this rest.

Again he limiteth a certain day,.... Since the seventh day of the creation was a day of rest which God entered into, and not man; and since the land of Canaan was a typical rest, which the unbelieving Israelites did not enter into, because of unbelief; and yet there must be persons, and there must be a time for them to enter into the true rest which God has left a promise of; therefore he has limited, fixed, and appointed a certain day, the Gospel dispensation, for believers to enter into it:
saying in David; or by David, who was the penman of the 95th psalm, as may be learned from hence; and this is agreeably to, and confirms a rule which the Jews give, that those psalms which are without a title were written by David (g); the Spirit of God spake in him and by him, and plainly pointed out another day of rest from the above mentioned:
today, after so long a time; as two thousand five hundred years from the first seventh day to the time of Moses, and five hundred years from the times of Moses and Joshua, to his:
as it is said; the Alexandrian copy reads, "as it is before said", or, "above said", as the Vulgate Latin, and Syriac versions; that is, in Psalm 95:7 before cited, Hebrews 3:7
today if you will hear his voice, harden not your hearts; See Gill on Hebrews 3:7, Hebrews 3:8.
(g) Aben Ezra & Kimchi Praefat. in Tillim.

Again--Anew the promise recurs. Translate as the Greek order is, "He limited a certain day, 'To-day.'" Here Paul interrupts the quotation by, "In (the Psalm of) David saying after so long a time (after five hundred years' possession of Canaan)," and resumes it by, "as it has been said before (so the Greek oldest manuscript, before, namely, Hebrews 3:7, Hebrews 3:15), To-day if ye hear His voice," &c. [ALFORD].

After so long a time - It was above four hundred years from the time of Moses and Joshua to David. As it was said before - St. Paul here refers to the text he had just cited.