When I gave all diligence to write unto you: the apostle here declares the first cause of his writing to them, viz. his own inclination and readiness, according to the duty of his place, (as an apostle), so to do: q. d. Being of myself willing, and earnestly desirous to promote your welfare, when absent from you, by writing unto you.
Of the common salvation; i.e. those things which concern the salvation of us all in common, or that salvation which is common to us all; there being but one salvation for all believers, and one way to it.
It was needful for me to write unto you, and exhort you; the second reason of his writing, viz. the necessity of it, in respect of the danger they were in, as follows, Jud_1:4.
That ye should earnestly contend; by constancy in the faith, zeal for the truth, holiness of life, mutual exhortation, prayer, suffering for the gospel, &c.; against those that would pervert the gospel.
For the faith; the doctrine of the gospel; faith is taken for the object of faith.
Which was once; either, once for all, because it was delivered by all the apostles as the only unchangeable rule of governing their lives, and obtaining salvation, to which nothing is to be added, and from which nothing is to be taken away; or it implies, that it was therefore delivered to them that they might never forsake it, and that if they do, they miss of their salvation, as being never like to have another way made known to them.
Delivered unto the saints; viz. by God, not invented by men.