The following is an excerpt from The MacArthur New Testament Commentary on Jude 3.

Jude initially made every effort to write regarding the common salvation he shared with his readers. Effort (spoude) connotes hastening or speed, and could mean Jude hurried in vain to write, or that he tried hard but could not complete what he originally planned to say. Whatever the case, the presence of false teaching restrained him, impressing him with the urgent need to call the church to battle. His initial notion was to speak positively of the shared blessings of salvation. But that very salvation was under assault by apostates, hence his change of subjects.

Like Paul, who wrote to the Corinthians, “For necessity is laid upon me; yes, woe is me if I do not preach the gospel” (1 Cor. 9:16, nkjv), Jude felt the necessity—a heavy burden or mandate—to write. Agcho, the root of the noun rendered necessity, means literally “compress.” Jude recognized that he was a watchman for the truth (cf. Ezek. 3:16–21) who could not simply watch in silence as his readers slipped into error. His fervent passion for sound doctrine, especially regarding the gospel, made even the thought of false teaching a heavy burden on his heart (cf. 2 Cor. 11:28). And he and his readers would not be able to share a common salvation if they lost the gospel.

Jude also had a deep love for his readers—meaning that he was dedicated to their spiritual well-being. Accordingly his tone conveyed a genuine care similar to that of Paul, who wrote to the Ephesian elders: “Therefore be on the alert, remembering that night and day for a period of three years I did not cease to admonish each one with tears” (Acts 20:31; cf. Col. 1:29).

Jude could not resist appealing (parakaleo, “exhorting, encouraging”) to his readers that they contend earnestly for the faith. The powerful expression contend earnestly translates a present infinitive (epagonizomai) and stresses the need to defend the truth continually and vigorously (cf. 1 Tim. 1:18; 6:12; 2 Tim. 4:7). It is a compound verb from which the English agonize is transliterated. From Jude’s day until now, true believers have always had to battle for the purity of the salvation gospel.

In referring to the faith, Jude is not speaking of a nebulous body of religious doctrines. Rather, the faith constitutes the Christian faith, the faith of the gospel, God’s objective truth (i.e., everything pertaining to our common salvation). It is what Luke wrote about in Acts 2:42, noting that the early believers “were continually devoting themselves to the apostles’ teaching” (cf. 1 Cor. 15:1–4; 2 Thess. 3:6). Paul admonished Timothy to protect that faith: “Retain the standard of sound words which you have heard from me, in the faith and love which are in Christ Jesus. Guard, through the Holy Spirit who dwells in us, the treasure which has been entrusted to you” (2 Tim. 1:13–14; cf. 1 Tim. 6:19–20).