Indeed, some learned Christians are so sensible of the insuperable difficulties attending every attempt to reconcile that Epistle to the Doctrine of inspiration, or even to common sense, that they avoid the trouble, by denying that Paul could have been the author of such a work, and attribute it to the same, or a similar, hand, with that which forged the marvellous Epistle ascribed to Barnabas.
The quotation brought forward in the Epistle to the Hebrews, to prove the abrogation of the Mosaic Law, and the substitution of a new one, is taken from Jer.xxxi.31, &c.--"Behold the days come saith the Lord, that I will make a new covenant with the house of Judah.

Not according to the covenant which I made with they fathers, in the day that I took them by the hand to bring them out of the land of Egypt, (which my covenant they brake, although I was an husband unto them, saith the Lord.) But this shall be the covenant that I will make with the house of Israel.

After those days saith the Lord, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people; and they shall teach no more every man his neighbour, saying know the Lord, for they shall all know me from the least of them unto the greatest of them, saith the Lord, for I will forgive their iniquity, and will remember their sins no more." Upon this passage the author of the Epistle observes "in that he saith 'a new covenant,' he hath made the first old;" and he sagely concludes " now that which decayeth, and waxeth old, is ready to vanish away!!" and takes the quotation to be a prophecy of the abolition of the old law, and the introduction of the Gospel Dispensation.
Now, I would observe on his reasoning, in the first place, that, allowing for a moment his interpretation of the prophecy to be correct, (i.e., that it signifies the abolishment of the old, and an introduction of a new law) the prophecy, at any rate, cannot refer to Jesus, or the Gospel; for so far from having been fulfilled in the time of Jesus, or his Apostles, it has not been fulfilled to this day; for certainly God has not yet made a new covenant with the Jews, to whom the prophecy refers, nor has he yet "put his law in their hearts;" nor "caused them to walk in it;" neither has he yet " forgiven their sins, or forgotten their iniquities," since they are even now suffering, the consequences of them.
I will now retract what I granted, and assert that the prophet did not mean an abolition of the Mosaic, and the introduction of a new, law; for though the prophet speaks of a new covenant, he says nothing of a new law; but on the contrary, asserts that this new covenant would be effectual to make them obey the law.

God promised to put his law within their hearts (not out of remembrance, as the catechisms say;) and in this alone this covenant differs from the one entered into at Mount Sinai.

For, then, though the law was given them, it was not "put within their hearts," but they were apt, to their own controul, to obey it, or not, being assured, however, that happiness should be the reward of obedience, and death and excision the punishment for revolt and disobedience.