Paul's prayer for Christians is that the God of peace would sanctify them completely (1 Thess. 5:23).
His assurance is that 'he is faithful, who will do it' (v. 24). From this prayer we learn firstly, that the one who sanctifies us is God. As God gave us our beings, so he gives us our holiness. It is not by nature but by grace that we are made holy. Secondly, we learn that the one who sanctifies us is emphatically declared to be God himself. If God does not do it, no one else will. And thirdly, we learn that the one who sanctifies us is the 'God of peace' (Rom. 15:33; 16:20; 2 Cor. 13:11; Phil. 4:9; Heb. 13:20).
Sanctified by the God of peace
Sanctification is a fruit of that peace with God which he has made and prepared for us by Jesus Christ. 'God was in Christ reconciling the world to himself' (2 Con. 5:19).
God, by sanctifying our natures, keeps that peace which we have with him. It is holiness that keeps up a sense of peace with God, and prevents those spiritual sins which still tend to break out from the corrupt nature still left within us. So God, as the author of our peace, is also the author of our holiness.
He will sanctify us completely, that is, entirely. No part of us will be left sinful or under the power of sin. Our whole nature is the subject of this work. He will make holy every part of us. And this work will eventually be perfected.
Paul prays, first, that our whole natures, our entire spirit, soul and body should be sanctified, and then he prays that we should be preserved blameless in the peace of God at the coming of our Lord Jesus Christ. This, he would have Christians know, is the whole purpose of our sanctification.
Sanctification the Spirit's work
Sanctification, then, is the direct work of the Holy Spirit on our whole nature. It proceeds from the peace made for us by Jesus Christ. By this peace with God through Jesus Christ, we will be preserved blameless, or be kept in a state of grace and continual acceptance with God, according to the terms of his covenant, to the end.
But before we go any further we must realize that there are two sorts of sanctification. There is that sanctification by which people or things are dedicated, consecrated, or set apart for the service of God, by his appointment. Such persons or things are in Scripture called holy. There is also that sanctification which is a ruling principle of holiness imparted to our natures, resulting in a life of holy obedience to God. We are now considering the second sort.
What, then, is holiness? Holiness is nothing but the implanting, writing and living out the gospel in our souls (Eph. 4:24). The gospel is the 'truth which is according to godliness' ('Titus 1 :1). Jesus prayed, 'Sanctify them by your truth. Your word is truth' (John 17:17). This is 'the truth that shall make you free' (John 8:32).
Evangelical holiness is a fruit wrought in us by the Spirit of sanctification. Holiness, therefore, is a mystery to fleshly reason (Job 28:20-23,28; 1 Cor. 2: 11, 12).
Believers are often ignorant of true holiness. They do not fully understand its true nature, its origin or the fruits it produces. It may seem strange that though all believers are made holy they should not understand what is wrought in them and what abides in them.
But do we understand our own creation (Psa. 139:13-16) This work of holiness in us is wonderful. It is a supernatural work and is known only by supernatural revelation.
We must not be deceived by a false holiness. Holiness is not just a reformed life.
Holiness is not only for this life, but goes on with us into eternity and glory. Death has no power to destroy holiness. The activities of holiness are indeed momentary and transient, but their fruits last forever in their reward (Rev. 14:13; Heb. 6:10). Holiness lasts forever and enters into glory with us (1 Cor. 13:8).
Holiness reveals something of the spiritual and heavenly glory even in this world (2 Cor. 3:18). The true believer is 'all glorious within' (Psa. 45:13). Holiness, then, is a glorious work of the Holy Spirit.
God's purpose in the gospel
God's first purpose in and by the gospel is to glorify himself, his wisdom, goodness, love, grace, righteousness and holiness by Jesus Christ, and that forever (Eph. 1:5, 6). But the gospel simultaneously reveals the love and grace of God to lost sinners which is brought to us by the mediation of Jesus Christ. And it is only by Jesus Christ as God incarnate that God will be glorified and sinners saved (Acts26:18).
The purpose of the gospel is also to persuade men by the preaching of the truth and the encouragement of the promises to renounce their sins and all other ways of satisfying God, and to receive by faith that way of life and salvation which by the gospel is preached to them ( 2 Cor. 5:18-21; Col 1:25-28).
The purpose of the gospel is also to be the means of bringing and giving to sinners a right and title to that grace and mercy, that life and righteousness, revealed and offered to them by the gospel (Mark 16:16). It also reveals the way and means of imparting the grace and strength of the Holy Spirit to the elect, enabling them to believe and receive salvation (Gal. 3:2).
The purpose of the gospel is also to bring believers into union with Christ as their spiritual and mystical head and to fix their hearts and souls in faith, trust, confidence and love directly on the Son of God as incarnate, and as their mediator (John 14:1). It is to invite and encourage lost sinners to believe and accept the way of grace, life and salvation by Jesus Christ.
God requires us to be holy. He requires us to use constantly all those means he has given us by which holiness may be attained and increased.
What God requires
God does not require us to make atonement or satisfaction for our sins although that is what guilty condemned sinners think (Mic 6:6, 7). But in the gospel there is offered an unconditional free pardon for sins. No satisfaction or compensation is required by God to be made by them. Christ has made a full and perfect atonement for sins. Any attempt to atone for our own sins instead of trusting in Christ's atonement is to trample over the gospel (2 Cor. 5:18-21).
What God does require is for us 'to do justly, to love mercy and to walk humbly' with him (Mic. 6:8). He does not require us to make ourselves righteous in order to be justified, because we are freely justified by his grace (Rom 10:3, 4; 3:24, 28; 8:3, 4).
God does not require us to purchase or merit life and salvation for ourselves (Eph. 2:8, 9; Titus 3:5; Rom. 4:4; 6:23; 11:6; Luke 17:10). Nor does he require of us good works to atone for our sins or works of supererogation to atone for the sins of others (Luke 17:10; Gen. 17:1).
God has promised to sanctify us, to work this holiness in us; he does not leave us to do it by our own ability and power (Jer. 31:33; 32:39, 40; Ezek. 36:26, 27).
God commands us to be holy and he promises to make us holy.
The believer's response
What, then, should be our response to God's command to be holy? Our first response should be that we make this duty a matter of conscience because it comes to us with all God's authority. Holiness must arise from obedience or it is not holiness. Our second response must be to see how reasonable this command is. Thirdly, we must love this command because it is holy and just and good and because the things it requires are right, easy and pleasant to the new nature.
And what should be our response to the promise that God will make us holy? Firstly, we must remember our utter inability to obey the command to be holy. Then we must see that our sufficiency is in God. Secondly, we must adore that grace which has promised to do in us what we are unable to do ourselves. Thirdly, we must pray in faith, believing God's promise to make us holy, and look to him to supply us with all grace necessary to walk in holiness. Fourthly, we should pray specially for that grace to keep us holy in times of temptation and when called to carry out special and difficult duties.
Finally, we must never forget that it is the Holy Spirit who sanctifies all believers, and who produces all holiness in them. (Psa. 51:10-12; Ezek 11:19; 36:25-27; Rom. 8:9-14; 1 Cor. 6:11; 1 Pet. 1:2; Isa. 4:4; 44:3, 4; Titus3:4, 5).
Excerpt from the book The Holy Spirit by John Owen