Faithful is he that hath called you. As he has shewn by his prayer what care he exercised as to the welfare of the Thessalonians, so he now confirms them in an assurance of Divine grace. Observe, however, by what argument he promises them the never-failing aid of God -- because he has called them; by which words he means, that when the Lord has once adopted us as his sons, we may expect that his grace will continue to be exercised towards us. For he does not promise to be a Father to us merely for one day, but adopts us with this understanding, that he is to cherish us ever afterwards. Hence our calling ought to be held by us as an evidence of everlasting grace, for he will not leave the work of his hands incomplete. (Psalm 138:8) Paul, however, addresses believers, who had not been merely called by outward preaching, but had been effectually brought by Christ to the Father, that they might be of the number of his sons.

Faithful is he that calleth you - That is, your sanctification after all depends on him, and as he has begun a work of grace in your hearts, you may depend on his faithfulness to complete it; see the 1-Thessalonians 4:3 note; Philippians 1:6 note; 1-Corinthians 1:9 note.

Faithful is he that calleth you - In a great variety of places in his word God has promised to sanctify his followers, and his faithfulness binds him to fulfill his promises; therefore he will do it. He who can believe will find this thing also possible to him.

(14) (k) Faithful [is] he that calleth you, who also will (l) do [it].
(14) The good will and power of God is a sure strengthening and encouragement against all difficulties, of which we have a sure witness in our calling.
(k) Always one, and ever like himself, who indeed performs whatever he promises. And an effectual calling is nothing else but a right declaring and true setting forth of God's will: and therefore the salvation of the elect, is safe and sure.
(l) Who will also make you perfect.

Faithful is he that calleth you,.... Into the fellowship of his Son, and to his kingdom and glory, and who continues to do so, not only externally by his word, but internally by his Spirit and grace.
Who also will do it. Two things the apostle mentions as the ground of confidence that the above petition, would be heard and answered; that is, that God would wholly sanctify them, and preserve the whole of them blameless to the coming of Christ; and they are the faithfulness of God, and the effectual calling of his saints. God is faithful to his word, his covenant and promises; he has promised to sanctify and cleanse his people from all their sins, and to preserve them safe to his kingdom and glory; agreeably the Arabic version renders this last clause, "and will execute his promise": and the effectual calling is a sure pledge of glorification; whom God calls he justifies and glorifies; as sure as he gives grace, he will give glory; and whom he calls to his eternal glory, he will make perfect, stablish, strengthen, and settle. The Complutensian edition reads, "who also will make your hope firm"; that is, with respect to the above things.

Faithful--to His covenant promises (John 10:27-29; 1-Corinthians 1:9; 1-Corinthians 10:23; Philippians 1:6).
he that calleth you--God, the caller of His people, will cause His calling not to fall short of its designed end.
do it--preserve and present you blameless at the coming of Christ (1-Thessalonians 5:23; Romans 8:30; 1-Peter 5:10). You must not look at the foes before and behind, on the right hand and on the left, but to God's faithfulness to His promises, God's zeal for His honor, and God's love for those whom He calls.

Who also will do it - Unless you quench the Spirit.