CHAPTER XIII—Of Sanctification
1. They, who are once effectually called, and regenerated, having a new heart, and a new spirit created in them, are further sanctified, really and personally, through the virtue of Christ’s death and resurrection, (1 Cor. 6:11, Acts 20:32, Phil. 3:10, Rom. 6:5–6) by His Word and Spirit dwelling in them, (John 17:17, Eph. 5:26, 2 Thess. 2:13) the dominion of the whole body of sin is destroyed, (Rom. 6:6,14) and the several lusts thereof are more and more weakened and mortified; (Gal. 5:24, Rom. 8:13) and they more and more quickened and strengthened in all saving graces, (Col. 1:11, Eph. 3:16–19) to the practice of true holiness, without which no man shall see the Lord. (2 Cor. 7:1, Heb. 12:14)
The Westminster Confession of Faith (Oak Harbor, WA: Logos Research Systems, Inc., 1996).
This is an assurance that many popular evangelists do not have themselves and cannot promise to their hearers. Yes, they insist on assurance, but it is not the assurance that the Bible teaches. These evangelists, the ones I have in mind, are Arminians. They do not believe in the perseverance of the saints, or, as they call it, eternal security. They claim to be very sure that they are saved now; but they are not sure that they will be saved tomorrow or next week. If they die tonight, they will be in Heaven immediately. But if they should live a while longer, they might fall into sin, fall from grace, and then they would be eternally lost. But they are very sure just now. Their mentality is hard to understand. How can anyone be very happy if he thinks he has an eternal life that is so little eternal that it might end next week? How can such a person look to the future with equanimity and confidence if he is so unsure of Heaven? Such an evangelist might as well be a Romanist. They talk about being born again, about regeneration; but the kind of regeneration they preach is something that a man must experience as many times as he falls from grace. To be really saved, i.e., to get to Heaven, one must be born again over and over again. Their hope therefore is one that can easily disappoint. These preachers often talk quite a lot about the Holy Spirit; but they deny to the Spirit the power to give a man eternal life. By eternal I mean eternal; not a life that ends in the near future. Thus they do not have assurance; nor do they preach the Gospel, for the Gospel promises at least the possibility of assurance. It promises, not the mere possibility of eternal life; it promises eternal life.
Gordon H. Clark (2013-03-04T05:00:00+00:00). What Is The Christian Life? (Kindle Locations 3353-3364). The Trinity Foundation. Kindle Edition.
The short answer is that assurance of salvation proceeds from obedience that is founded upon the promises of God in Scripture, including justification by faith alone. And you will notice that Dr. Gordon H. Clark indirectly implies that Arminians do not preach the Gospel. Would that more Reformed ministers and scholars would say these things more straightforwardly.
Sanctification is not the basis for saving faith. But is a necessary result of regeneration. Those who have no change of habits cannot have assurance of salvation (Hebrews 12:14). On the other hand, those who endeavor to walk by faith and to obey the information revealed in God's written Word can be most certain that God will never let them fall away.
What is the communion in grace which the members of the invisible church have with Christ?
The communion in grace which the members of the invisible church have with Christ, is their partaking of the virtue of his mediation, in their justification, (Rom. 8:30) adoption, (Eph. 1:5) sanctification, and whatever else, in this life, manifests their union with him. (1 Cor. 1:30)
Wherein do justification and sanctification differ?
Although sanctification be inseparably joined with justification, (1 Cor. 6:11, 1 Cor. 1:30) yet they differ, in that God in justification imputeth the righteousness of Christ; (Rom. 4:6 ,8) in sanctification of his Spirit infuseth grace, and enableth to the exercise thereof; (Ezek. 36:27) in the former, sin is pardoned; (Rom. 3:24–25) in the other, it is subdued: (Rom. 6:6,14) the one doth equally free all believers from the revenging wrath of God, and that perfectly in this life, that they never fall into condemnation (Rom. 8:33–34) the other is neither equal in all, (1 John 2:12–14, Heb. 5:12–14) nor in this life perfect in any, (1 John 1:8,10) but growing up to perfection. (2 Cor. 7:1, Phil. 3:12–14)
The Westminster Larger Catechism: With Scripture Proofs. (Oak Harbor, WA: Logos Research Systems, Inc., 1996).