This is an advertisement very needful; because many are apt to skip over the lesson concerning the means (that will fill up this whole treatise) as superfluous and useless. When once they know the nature and excellency of the duties of the law, they account nothing wanting but diligent performances; and they rush blindly on immediate practice, making more haste than good speed. They are quick in promising, ‘All that the Lord has spoken, we will do,’ (Exod. 19:8), without sitting down and counting the cost. They look on holiness as only the means of an end, of eternal salvation: not as an end itself, requiring any great means for attaining the practice of it. The enquiry of most, when they begin to have a sense of religion, is ‘What good thing shall I do, that I may have eternal life?’ (Matt. 19:16); not, ‘How shall I be enabled to do anything that is good?’ Yea, many that are accounted powerful preachers spend all their zeal in the earnest pressing the immediate practice of the law, without any discovery of the effectual means of performance – as if the works of righteousness were like those servile employments that need no skill and artifice at all, but industry and activity. That you may not stumble at the threshold of a religious life by this common oversight, I shall endeavor to make you sensible that it is not enough for you to know the matter and reason of your duty, but that you are also to learn the powerful and effectual means of performance before you can successfully apply yourselves to immediate practice. And, for this end, I shall lay before you the considerations following.

…Sanctification, by which our hearts and lives are conformed to the law, is a grace of God communicated to us by means, as well as justification, and by means of teaching, and learning something that we cannot see without the Word (Acts 26:17, 18). There are several things pertaining to life and godliness that are given through knowledge (2Peter 1:2, 3). There is a form of doctrine made use of by God to make people free from sin, and servants of righteousness (Rom. 6:17, 18). And there are several pieces of the whole armor of God necessary to be known and put on, that we may stand against sin and Satan in the evil day (Eph. 6:13). Shall we slight and overlook the way of sanctification, when the learning the way of justification has been accounted worth so many elaborate treatises?

—Walter Marshall (1628–90), The Gospel Mystery of Sanctification (An interview on this work. More resources on sanctification)