3. Design of the Epistle (compare Jude 20, 21).
      all diligence-- (2Pe 1:5). As the minister is to give all diligence to admonish, so the people should, in accordance with his admonition, give all diligence to have all Christian graces, and to make their calling sure.
      the common salvation--wrought by Christ. Compare Note, see on 2Pe 1:1, "obtained LIKE precious faith," This community of faith, and of the object of faith, salvation, forms the ground of mutual exhortation by appeals to common hopes and fears.
      it was needful for me--rather, "I felt it necessary to write (now at once; so the Greek aorist means; the present infinitive 'to write,' which precedes, expresses merely the general fact of writing) exhorting you." The reason why he felt it necessary "to write with exhortation," he states, Jude 4, "For there are certain men crept in," &c. Having intended to write generally of "the common salvation," he found it necessary from the existing evils in the Church, to write specially that they should contend for the faith against those evils.
      earnestly contend--Compare Php 1:27, "striving together for the faith of the Gospel."
      once, &c.--Greek, "once for all delivered." No other faith or revelation is to supersede it. A strong argument for resisting heretical innovators (Jude 4). Believers, like Nehemiah's workmen (Ne 4:17), with one hand "build themselves up in their most holy faith"; with the other they" contend earnestly for the faith" against its foes.
      the saints--all Christians, holy (that is, consecrated to God) by their calling, and in God's design.

JFB.