Hebrews 7:18

For there is a disannulling of a foregoing commandment because of its weakness and unprofitableness.

This turns attention to the very nature of the Levitical system of which that priesthood was the support and center. It was not of caprice that God annulled the old covenant, for it deserved to be annulled because of its weakness and unprofitableness. God had never considered the Levitical system to be complete, final or efficacious in itself; but "it was added because of transgression, until the seed should come to whom the promise hath been made") Galatians 3:19). The law expired, therefore, by limitation, when Jesus was revealed as that "seed" so long anticipated. The weakness and unprofitableness of that foregoing commandment refers to the whole system of Moses; and Macknight explained the weakness of it thus,

The weakness of the law in reforming sinners arose from this, that while it required perfect obedience to all its precepts under penalty of death, it gave the Israelites no encouragement to obey, either by promising them the assistance of God's Spirit to enable them to obey, nor by giving them assurance of pardon upon their repentance in case of failure. The only source from which the Israelites derived their hope of these things was the covenant with Abraham.[22]

In connection with the allegation by some that "the law" here spoken of as "annulled" or the commandment said here to be abrogated was merely the "ceremonial" of Moses' law, it should be pointed out that the weakness and unprofitableness of that system were lodged more in the moral than in the ceremonial element of it. The efficiency and strength of that law, as far as providing and regulating a priesthood are concerned, were absolutely superlative. Paul said that "If there had been a law given that could make alive, verily righteousness would have been of the law" (Galatians 3:21). In those words Paul plainly indicated that the law went as far as it was possible for any law to go toward making people righteous; and yet it left them dead; and therefore, the weakness and unprofitableness of it have to be sought in the very portion of it called the moral law, and principally there. And why weak? Because it dealt with overt actions, rather than inward desire. When Jesus was said by Paul to have taken the "handwriting of ordinances" out of the way, nailing it to the cross, thus making a show of them openly, "triumphing over them in it," of what could he have been speaking if not the moral law, along with all the rest of it, and particularly of the very Decalogue itself?. Certainly not of the rules and specifications bearing upon the commissioning of Levitical priests. And how did Jesus triumph over them openly in it (the Decalogue)? - by showing that people could keep the letter of it and still be guilty and impure in heart. All efforts, then, to restrict the weakness and unprofitableness, here mentioned, to the ceremonial structure of the Mosaic system must be rejected as foreign to the teachings of the scriptures. In this connection, please see under Hebrews 7:11, and also Matthew 5 where the triumph of the Lord over the Pharisees in the Decalogue is dramatically documented.