What does it mean to be saved by sanctification and believing the truth? 2 Thessalonians 2:13

The declaration that "Salvation is by grace alone, through faith alone, in Christ alone" is something of the cry of Reformed Protestants, so this answer is from a Reformed Protestant perspective.

The verse being asked about says in full:

"But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth." 2 Thessalonians 2:13 A.V.

To be sanctified is to be set apart (by God) for his holy purpose. It is vital to salvation because God is not going to save an unsanctified (i.e an unholy) person. "Without holiness, no-one will see the Lord", it says in Hebrews 12:14. As we are not holy, but are sinners, we require God to deal with our unholiness, which he has done by what Jesus achieved at the cross. There divine justice was met by Jesus bearing the punishment for sin that ought to have been poured out on us. Sin was dealt with by God stepping in to make provision for unmerited pardon. Now all those who put faith in that gracious provision will be freely forgiven and cleansed from their sin. It is faith in the finished work of Christ that is needed for grace to be poured out on helpless sinners who realise they have been saved by grace alone, through faith alone, in Christ alone.

They have believed the truth of the gospel of Christ (Mark 1:1). That is not mere intellectual assent; it is putting faith in what they now know Jesus achieved at the cross - trusting in what he did to save them. As it says in Galatians 3:11 and Romans 1:17, "the just shall live by faith" (quoting Habakkuk 2:4). Their faith is demonstrated in the way they now live.

This means that 2 Thessalonians 2:13 is referring to the power of the Holy Spirit's transforming sanctification in the believer in Jesus. This is confirmed by the same points in Ephesians 1:12-14 which speaks of Christians who

"...first trusted in Christ. In whom ye also trusted, after that ye heard the gospel of your salvation: in whom also that ye believed, ye were sealed with that holy Spirit of promise, which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory." A.V.

The gospel of Christ, which is the gospel of sanctification and salvation, is believed, and the Holy Spirit guarantees their salvation to them, as explained in Romans chapter 8. This is all pure grace - unmerited, and at God's initiative.

That is a basic Reformed Presbyterian understanding of how people are saved by the sanctification of the Spirit and believing the truth.

Scripture speaks of us as having been saved (in God's economy), being saved (experientially through sanctification), and that we will be saved (ultimately). 1 Peter 1:2 says much the same thing as the verse in question:

Elect according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ

Election is something that God has foreseen from before creation and, having foreseen and then created, election is a finished process in God's economy. However, being temporal, it is a finished process that we experience in the present:

Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life. - John 5:24

We hear presently and believe presently which affords us present unending life, deliverance from condemnation both present and future, and a change of spiritual make up ... dead to living.

For it is by grace you have been saved

Since salvation is provided in the face of personal sinfulness it is by unmerited favor only. There is no sense in which a sinner deserves salvation, can earn salvation, or can repay the provider. Salvation is given here in the perfect tense which indicates an action completed in the past with effects in the now and it is given in the passive voice which indicates that the subject receives the action rather than performs it. If we hear His voice and believe in the One who sent Him we have been saved.

through faith

Faith is the vehicle by which this completed salvation is delivered to us in the here and now: It is through faith. Here is part of a description of the definition of faith (pistis) which is quite different from the 'blind faith' notion we so often hear about:

Our English word "faith" (from the Latin word fides, meaning trust or confidence) is in modern times almost wholly reserved for religious sentiments, and if it is used in a secular way it denotes the accepting of something for which no evidence exists. Hence we take something on faith, or have faith in something untried. In New Testament times there was no such thing as "faith" as we know it, and with the word πιστις (pistis) people referred to the mental substance that results from intelligent and reasonable inquiry or instruction; the mind's response to valuable information. It described sureness in every way, from sureness about salvation to sureness about how to make a club sandwich. Someone's pistis did not denote one's religious or political leanings but comprised everything a person was certain of, on whatever level and whatever field, without distinction between scientific, artistic and religious certainties or even little practical certainties to help with daily chores. Our noun πιστις (pistis) denotes the foundation of all a person's willful activities, and the whole spectrum of a person's acts and conduct; not a dusty library of dogmas but the open-source social software upon which a person's daily life runs.

So the vehicle through which a past completed salvation is delivered to us is a sure and certain foundation for all of our willful activities: Faith. "He that hath received his testimony hath set to his seal that God is true." - John 3:33

and that not of yourselves; it is the gift of God

The certainty Christians have which makes up the foundation of our willful activities is not something that we simply possess by being human and it is not exercised in the absence of 'proof'. It is given to us by God as an internal testimony of the salvation that has already been accomplished:

He that believeth on the Son of God hath the witness in himself: he that believeth not God hath made him a liar; because he believeth not the record that God gave of his Son. And this is the record, that God hath given to us eternal life, and this life is in his Son. He that hath the Son hath life; and he that hath not the Son of God hath not life. - 1 John 5:10-12

And it is the Holy Spirit. The Spirit of God. The Spirit of Christ. This is given to us by God when we hear the voice of the Son and believe that God sent Him. When we cast aside every whim of whatever worldview under which we currently operate and believe in the Lord Jesus Christ, making the salvation of God the new worldview under which we operate, the Holy Spirit is sent into our hearts to make it certain.

This where sanctification lies (through sanctification of the Spirit - 1 Peter 1:2). This Spirit now lusts against our flesh and our flesh lusts against the Spirit as the Spirit works within us to conform us into the image of Christ. This is the here and now, present experience of an already completed salvation which is being delivered to us by grace (unmerited favor) through faith (the mind's response to valuable information).

You must be born again or you do not have the Spirit of Christ and are not entered into this sanctification process. No amount of religious activity or zeal can get you there. You must be born again.

In view of what God has done ... and demonstrated that He has done ... and promised that He will do ... offer your body as a living sacrifice (change your mind, believe, and ask Christ to enter you (the temple of the Holy Spirit) and take the throne of your heart as King), be transformed by the renewing of your mind (cooperate with the Spirit to strengthen and perfect certainty in the foundation of your willful activities), and you will, ultimately, have your salvation consummated. He is faithful who has called you. He will do it.

"For this is the will of God, even your sanctification"