Beloved, when I gave all diligence to write unto you of the common salvation, it was necessary for me to write unto you, and exhort you that you should earnestly contend for the faith which was once delivered unto the saints.
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Aquinas Study Bible
AD 2017
common salvation: That is, of that salvation which was shared by him and them. For indeed the salvation, faith, and love of Christ are one and common to all the elect. (St. Bede)
faith once delivered to the saints: That is, apostolic teaching, (St. Irenaeus Frg. 36) which was left with the Church. (Bishop John McEvilly)
Being very solicitous to discharge my duty of an apostle, in writing and instructing you in the common concern of your salvation, I judge it necessary at present to write this letter, to exhort you to contend earnestly, and stand firm in the Christian faith. (Witham)