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.