For if that first covenant had been faultless, then should no place have been sought for a second. "For" introduces this sentence as a reason for what has been already said; i.e. for a better covenant having been spoken of.
The expression might be objected to by Hebrew readers as implying imperfection in the original Divine covenant. "Nay," says the writer, "it was imperfect, it was not faultless; for prophecy itself declares this."
Should it be further objected that in the prophecy it is not the old covenant itself that is found fault with, but the people for not observing it, the answer would be that the remedy for their non-observance being the substitution of a new one that would answer its purpose better, some imperfection in the old one is implied.
This is indeed the very point of this verse. If it be asked, further, how faultiness in the old covenant is compatible with the view of its Divine origin, the answer is abundantly supplied in St. Paul's Epistles.
His position constantly is that the Mosaic Law, though in itself "holy, just, and true," and adequate to its purpose, was still imperfect as a means of justification. It was but a temporary dispensation, with a purpose of its own, intervening between the original promise to Abraham and the fulfillment of that promise in Christ.
Thus it is no derogation to itself or to its Author to charge it with "weakness and unprofitableness" for a purpose it was never meant to answer.