“All of these blessings that come to you in the gospel of Christ are the things in which all of your spiritual life and happiness consist. If you have them, you already have everlasting life. You do not live a holy life in order to gain and earn everlasting life. You already have it. The everlasting life God has given you in the gospel is what enables you to live a holy life.
The terms of the law make holiness the cause of eternal life.
The terms of the law are completely contrary to this way of living a holy life. The terms of the law place the practice of holiness before life. The terms of the law make holiness the cause of eternal life. ‘If you live a holy life, you will earn eternal life.’ Moses describes the terms this way: ‘The one who does these things shall live by them’ (Roman 10:5). By these terms, you must do the holy duties that are commanded before you have any right to receive the life that is promised to you. Under these terms, you also have to live a holy life without any of the gospel blessings I just mentioned, because you have not yet earned the right to have these blessings.
This viewpoint turns the gospel on its head.
In this view, the true means of holiness—the principles of life implanted in you by the gospel— are no longer the cause of holiness. Rather, they become the effect and the fruit of holiness. Under this view you cannot receive them until you work for them! You can see how under this view you will never be able to expect any true holiness. True holiness is destroyed because everything that produces true holiness has been taken away. This is why the apostle Paul says that the way of salvation by the works of the law makes faith empty, and the promises of no effect. The way of salvation by works frustrates the grace of God, as if Christ died in vain. It makes Christ to be of no value to you, as to those who have fallen from grace (Romans 4:14; Galatians 2:21; 5:2, 4).”
Walter Marshall, The Gospel Mystery of Sanctification, pp. 90-91