[I'm going to beg Boston's pardon and give you a paraphrase rather than a direct quote (of the passage mentioned here).]
Reasons why people remain under the broken covenant of works
(a) It is natural to us. This covenant being made with Adam and with us in him, it is ingrained in the hearts of all men naturally. There are impressions of it to be found in the hearts of all, among the ruins of the fall. The law as a covenant of works was the first husband that human nature was wedded to, and so it is still natural to us to cling to it. See it:
- in men left to the swing of their own nature; they all go this way in their dealing with God for life and favour. All false religions agree in this, that it is by doing that man must live, although they hugely differ in the things that are to be done for life. … Consider all unrenewed men whatsoever, even those who have the knowledge of life and salvation and profess to have the hope of life and salvation in the way of the covenant of grace; yet in practice they stumble at this stumblingstone, Matt 5:3.
- in those who are awakened and convinced of sin, and are in moral seriousness seeking to know what course they should take to be saved, and putting all effort into achieving this aim. They all take this principle for granted, that it is by doing that they must obtain life and salvation: Matt 19:16, ‘What good thing shall I do that I may inherit eternal life?’ Luke 10:25, ‘What shall I do to inherit eternal life?’ And this remains true even when they are pricked to the very heart, and the law as the covenant of works has wounded them to the very soul. They never think of a divorce from the law so that they may be married to Christ; but their concern is how they shall please the old husband so as to be saved from wrath.
- in the saints, who are truly married to Jesus Christ. What hankering there is after the first husband, how great the remains of a legal spirit, how hard it is for them to forget their father Adam’s house. There is a tendency to deal with God in the way of giving so much duty for so much grace and favour with God; even the best have to strive with it continually.
(b) The way of the covenant of works is most agreeable to the pride of man’s heart. A proud heart will rather serve itself with the less than stoop to live upon free grace. Rom 10:3, ‘For they being ignorant of God’s righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God.’
Man must be broken, bruised, and humbled, and laid very low, before he will embrace the covenant of grace. While a broken board of the first covenant will do men any service, they will hold by it, rather than come to Christ – like men who will rather live in a cottage of their own, than in another man’s castle.
To renounce all our own wisdom, works, and righteousness, and to cast away as filthy rags all those garments which we have been at so much pains to patch up, goes wholly against the grain with corrupt nature.
(c) The covenant of works is most agreeable to man’s reason in its corrupt state. If one had asked the opinion of the philosophers, concerning a religion which taught salvation by a crucified Christ, and through the righteousness of another, they would have said it was unreasonable and foolish, and that the only way to true happiness was the way of moral virtue. The rabbis would have declared it scandalous, and would have maintained that the only way to eternal life was by the law of Moses. To this day, many learned men cannot see the reasonableness of the gospel method of salvation, in opposition to the way of the covenant of works. This is why our forefathers are in effect looked upon as a parcel of well-meaning simple men, whose doctrine must be reformed over again, and rendered more agreeable to reason. Even unlearned men: how they dispute against the gospel method, against the promise, against their believing their welcome to Christ when they are so sinful and unworthy!
(d) People remain under the covenant of works through ignorance and insensibleness of the true state of the matter, as it now is. There is a thick darkness around Mount Sinai, so that they who live under the covenant of works see little other than what they see by the lightnings which flash out now and then. Hence they little know where they are, nor what they are.
- They do not understand the nature of the covenant of works. Any notion they have of it is lame and weak, without efficacy. They do not see how forcibly it binds to perfect obedience and satisfaction, how rigorous it is in its demands, and how it will abate nothing, even if a man were to work to the utmost of his power, and with cries and tears of blood seek forgiveness for the rest. They are not acquainted with the spirituality of the law, and the vast compass of the holy commandment: they stick too much in the letter of it, and narrow its demands, so that they may be the more likely to fulfill them.
- They are not conscious of their own utter inability for that way of salvation (by the terms of the covenant of works). They know they are off the way, and have wandered from God, but they hope they will get back to him again by repentance, while in the meantime their heart is a heart of stone and they cannot change it. They know they have sinned, and provoked justice against them, but they hope to be sorry for their sin, and to pray to God for forgiveness, and bear patiently any thing that God lays on them – while in the meantime they do not see that none of these things will satisfy God’s justice, which must have full satisfaction for every sin of theirs before they will see heaven. They know they must be holy, but they hope to serve God better than they ever have done, while in the meantime they do not consider that their work arm is broken, and that they can do nothing to purpose till they are saved by grace.
[Some way later, he begins to discuss this point:]
Christ’s fulfilling all righteousness, as the Surety and Representative of his people, is the grand and only condition of the covenant of grace
[But that will have to do for now - the post is already long enough.]