"Sanctify them through thy truth: thy word is truth" (John 17:17).
It is the truth that separates. Therefore all those who hold the truth in belief and action are separate from the world and the things of the world, and so can have no fellowship with the world. Those who would suggest that the brethren of Christ join in with the churches around us obviously do not understand God's word in its fulness and so do not have the truth. Christ states in his prayer in the previous verse "They are not of the world, even as I am not of the world". This being the case how can we fellowship the world when it hateth Christ (John 7:7John 7:7 7 The world cannot hate you; but me it hateth, because I testify of it, that the works thereof are evil.)?
The root word from which "sanctify" is translated in this verse is "hagios" which, says Strong's concordance, "It means specifically pure. But this may be only in a ceremonial sense, or it may have a moral signification. It sometimes describes freedom from impurities of the flesh" and therefore Christ is saying that the truth as contained in God's Word requires purity and separation from anything that might defile. It is hoped that a full explanation of the word "hagios" and its synonyms may be able to be provided at some point God willing.
1 Thessalonians 2:10 - "Ye are witnesses, and God also, how holily and justly and unblameably we behaved ourselves among you that believe:"
Paul declares that both the disciples and God are witnesses of the behaviour of himself, Silvanus and Timotheus whilst in the company of the Thessalonian ecclesia and in so doing uses three adjectives; holily, justly and unblameably. Any one of these would have done but would not have given any where near the emphasis the Spirit chose that Paul should use regarding their behaviour, but it also wouldn't have covered the breadth of their good example. The exhortation to each of us to follow this example is so obvious that it does not require explanation here.The first word "holily" is the Greek hosios and pronounced "hos-ee-oce'". This word occurs only once in Scripture and means "piously, holily". It originates from hosios (the same when transliterated into English, but pronounced "hos'-ee-os") which means "undefiled by sin, free from wickedness, religiously observing every moral obligation, pure holy, pious". The word translated "unblameable" is used again in 1 Thessalonians 3:13 - "To the end he may stablish your hearts unblameable in holiness before God, even our Father, at the coming of our Lord Jesus Christ with all his saints."
The exhortation to each of us to follow this example is so obvious that it does not require explanation here.
...the saints should be separate from the world, having nothing to do with it's sinful ways; for this is the world that God will destroy with the return of Christ to the earth
The Spirit through Paul uses the same word used in 1 Thessalonians 2:101 Thessalonians 2:10 Ye are witnesses, and God also, how holily and justly and unblameably we behaved ourselves among you that believe: for "unblameable" but the word "holiness" comes from a completely different word (hagiosune) used three times in Scripture; once for God's majesty and holiness, and twice (including here) to refer to "moral purity". The saint therefore should be morally pure, emulating God's holiness, and since God cannot abide sin or come into contact with it or anything that is defiled by it, the saints should be separate from the world, having nothing to do with it's sinful ways; for this is the world that God will destroy with the return of Christ to the earth, when God's Kingdom will replace the kingdom of men.
1 Thessalonians 4:3, 7 - "For this is the will of God, even your sanctification, that ye should abstain from fornication: ...For God hath not called us unto uncleanness, but unto holiness."
The words "sanctification" and "holiness" in these two verses are translated from the same Greek word and can be equated to the Hebrew qodesh, meaning separation. We have been called to separation from the world and its evil influences for it is a world that does not know Yahweh, and does not want to know the Creator and His ways. Jesus said in his prayer recorded in John 17 "I have given them thy word; and the world hath hated them, because they are not of the world, even as I am not of the world." And how can we love the world that hates Christ?If we love the world and are loved of the world then we are not true disciples and servants of Christ, for "If ye were of the world, the world would love his own: but because ye are not of the world, but I have chosen you out of the world, therefore the world hateth you" (John 15:19). And how can we love the world that hates Christ? Surely, if we do either of these then are we not unfaithful to our master, and deserve what the world gives us? If we wish to "inherit the kingdom prepared for us from the foundation of the world" (Matthew 25:34) then we must be faithful and profitable servants and worthy of the grace that will be bestowed on all those that love his appearing (2 Timothy 4:82 Timothy 4:8 Henceforth there is laid up for me a crown of righteousness, which the Lord, the righteous judge, shall give me at that day: and not to me only, but unto all them also that love his appearing.).
This will require effort and stamina to last the distance; for we do not know when our Master will return from Heaven to judge the saints but one thing we can be certain of is that it will not be that far in the future. We should therefore maintain a state of readiness, watching the signs around us, so that unlike the world Christ will not come on us as a thief in the night as he threatened to do to the ecclesia at Sardis (Revelation 3:2-3Revelation 3:2-3 2 Be watchful, and strengthen the things which remain, that are ready to die: for I have not found thy works perfect before God. 3 Remember therefore how thou hast received and heard, and hold fast, and repent. If therefore thou shalt not watch, I will come on thee as a thief, and thou shalt not know what hour I will come upon thee.).