Salvation here means the whole work of men's salvation. In a general sense it is the redemption of mankind, the whole fullness of God's counsel concerning the redemption of the world and the salvation of all men. The concept of common salvation embraces the salvation of all people without distinction, Jews and Gentiles alike, if only they turn to faith in Christ and enter His gracious Kingdom, the Church of Christ.
"I found it necessary to write to you": partly by the duty of an Apostle, as a herald of faith in Christ and of apostolic teaching (1 Cor 9:16), and partly in view of the danger threatening Christians from false teachers who had intruded into the Church (Jude 1:4; cf. 2 Tim 2:17).
Since the Apostles, teachers and guides of the whole Christian community, could not always be everywhere, while the Churches planted by them needed instruction, counsel, admonition, and teaching, it was necessary to write to them.
The Apostle's manner of speech is such that he seems first to have intended to set forth in detail the proclamation about Christ and the work of the world's salvation accomplished by Him. The present Epistle seems only a brief outline or note from that more extensive writing.
But perhaps it was not pleasing to the Holy Spirit that such an intention be fulfilled. Likewise, for unknown reasons, the Holy Spirit did not permit Paul and Timothy to preach the Gospel of Christ in Asia and Bithynia (Acts 16:6-7).
"Exhortation": more precisely from the original, as in the Slavonic text, "beseeching, asking, imploring," which is stronger than exhorting.
"To contend for the faith": to act with zeal, diligence, and effort, without weakening, overcoming every obstacle, and enduring struggle, suffering, and every kind of hindrance.
"For the faith": by faith here is meant the whole Gospel teaching, or all Christianity, both in its theoretical and practical aspects (cf. Acts 14:27; 1 Tim 4:1; Gal 1:23).
It was once and for all, without any alteration, delivered as the greatest treasure (1 Cor 11:23; 1 Cor 15:3; 2 Tim 1:12) from the Lord Jesus Christ through His holy Apostles and teachers of the Church to the saints, that is, to believing Christians (Heb 12:14).
"Once": a small word with the greatest significance. The faith delivered once must be preserved by believers in the Church forever, unto the ages of ages. It must be preserved without any alteration, without addition or omission, as it was taught by the Lord Himself (Gal 1:7-8; 2 Thess 2:15).