"In the Larger Catechism of the Westminster Assembly the question is asked, "What is sanctification?" To which the following answer is returned: "Sanctification is a work of God’s grace, whereby, they whom God hath before the foundation of the world chosen to be holy, are in time through the powerful operation of His Spirit, applying the death and resurrection of Christ unto them, renewed in their whole man after the image of God; having the seeds of repentance unto life and all other saving graces, put into their hearts, and those graces so stirred up, increased, and strengthened, as that they more and more die unto sin and rise unto newness of life."
Now far be it from us to sit in judgment upon such an excellent and helpful production as this Catechism, which God has richly blest to thousands of His people, or that we should make any harsh criticisms against men whose shoes we are certainly not worthy to unloose. Nevertheless, we are assured that were its compilers on earth today, they would be the last of all to lay claim to any infallibility, nor do we believe they would offer any objection against their statements being brought to the bar of Holy Scripture. The best of men are but men at the best, and therefore we must call no man "Father." A deep veneration for servants of God and a high regard for their spiritual learning must not deter us from complying with "Prove all things: hold fast that which is good" (1 Thess. 5:21). The Bereans were commended for testing the teachings even of the apostle Paul, "And searched the Scriptures daily whether those things were so" (Acts 17:11). It is in this spirit that we beg to offer two observations on the above quotation.
First, the definition or description of sanctification of the Westminster divines is altogether inadequate, for it entirely omits the most important aspect and fundamental element in the believer’s sanctification: it says nothing about our sanctification by Christ (Heb. 10:10; 13:12), but confines itself to the work of the Spirit, which is founded upon that of the Son. This is truly a serious loss, and affords another illustration that God has not granted light on all His Word to any one man or body of men. A fuller and better answer to the question of, "What is sanctification?" would be, "Sanctification is, first, that act of God whereby He set the elect apart in Christ before the foundation of the world that they should be holy. Second, it is that perfect holiness which the Church has in Christ and that excellent purity which she has before God by virtue of Christ’s cleansing blood. Third, it is that work of God’s Spirit which, by His quickening operation, sets them apart from those who are dead in sins, conveying to them a holy life or nature, etc."
Thus we cannot but regard this particular definition of the Larger Catechism as being defective, for it commences at the middle, instead of starting at the beginning. Instead of placing before the believer that complete and perfect sanctification which God has made Christ to be unto him, it occupies him with the incomplete and progressive work of the Spirit. Instead of moving the Christian to look away from himself with all his sinful failures, unto Christ in whom he is "complete" (Col. 2:10), it encouraged him to look within, where he will often search in vain for the fine gold of the new creation amid all the dross and mire of the old creation. This is to leave him without the joyous assurance of knowing that he has been "perfected forever" by the one offering of Christ (Heb. 10:14); and if he be destitute of that, then doubts and fears must constantly assail him, and the full assurance of faith elude every striving after it." - A.W. Pink, The Doctrine of Sanctification