By this he suggests that Timothy is able to take the place of the Apostle Paul himself, and that they can trust him and, in conscience, ought to listen to him as to the Apostle Paul himself. At the same time he showed that, by sending such an envoy, he has a very serious concern for them, one touching the most important aspects of their life, and thereby obliges them to full attention.
Saint Chrysostom says that Saint Paul praises Timothy in this way "not by exalting Timothy, but by honoring them through having sent them a coworker and minister of the Gospel. As though saying: drawing him away from affairs, we sent to you a minister of God and our fellow worker in the Gospel of Christ." The same is found in Oecumenius and Theophylact.
And we sent Timothy, our brother. Christians mutually called themselves brothers, as those born again from one Spirit in one font, united with the one Lord, and forming one Church. But in calling Timothy this, the Apostle wants to say that he is the person closest to him. They are bound by sincere love, and for them Timothy is the same as the Apostle himself.
He is a brother to whom the Apostle entrusts everything, and to whom they too may entrust themselves. For his brother is their brother.
A minister of God. This indicates Saint Timothy's special appointment by God Himself. A minister of God is the executor of God's special intentions, one entrusted by God to accomplish something for God's glory, in fulfillment of the plans of Providence. Such also was Tychicus, a faithful minister in the Lord (Eph 6:21; Col 4:7).
Apollos, and even Paul himself, are nothing but ministers, as the Lord gave to each (1 Cor 3:5).
And our fellow worker. This defines precisely Timothy's service to God. He has been appointed to be Saint Paul's fellow worker in the preaching of the Gospel. So he is also named in the Epistle to the Romans (Rom 16:21), where the Apostle calls himself one who serves God in his spirit in the Gospel of His Son (Rom 1:9). He gives the same name to Epaphroditus the Philippian (Phil 2:25).
To strengthen and comfort is the aim of the sending. Both words together embrace all the aspects on which Saint Timothy could and had to act. So also Silas and Judas, sent from Jerusalem to Antioch, comforted and strengthened the brethren with many words after the disturbances produced among them by zealots for the Jewish law (Acts 15:32).
The work of Saint Timothy was to strengthen still more those standing in the faith, to inspire the weak and those ready to waver toward steadfastness, and to comfort those grieving because of persecution, so that no one would fall and fall away. "The Thessalonians needed strengthening and consolation, so as not to fall away from the faith" (Theophylact).
If each word is considered separately, to strengthen means to confirm the spirit in its way of thinking and rules of life, and to give force to convictions received in faith and to resolve to live according to the demands of faith. "To strengthen, as those who are wavering" (Oecumenius). "Not to offer you a new teaching, but to strengthen you in the former teaching" (Theodoret).
To comfort means to calm the heart that is agitated, suffering, and sorrowing because of the troubles in which the Thessalonians had been placed on account of receiving the Gospel. Another time the Apostle wrote to them: "May the Lord comfort your hearts and establish you in every good word," that is, in thoughts and convictions, "and work," that is, in rules of life according to faith (2 Thess 2:17).
Concerning your faith may be referred only to comfort. The heart was in sorrow and distress because of the circumstances brought on by their believing. Because of this, the heart's sorrow cast a shadow upon the faith itself. By interpreting the encountered constraints in a manner worthy of God, Saint Timothy could comfort their hearts and thereby again restore the brightness of faith's countenance in the consciousness of the believers.
Concerning your faith means: with respect to your faith in these circumstances, which were caused by your believing.