Jude 1:4

For there are certain men crept in unawares who were before of old ordained to this condemnation, ungodly men, turning the grace of our God Into lasciviousness, and denying the only Lord God and our Lord Jesus Christ.

Here Jude not only gives his reasons for writing, but also for writing as he did; Beloved, as I was putting forth all earnestness to write to you concerning our common salvation, I felt constrained (instead) to write to you, admonishing you to contend for the faith which once for all has been committed to the saints. Jude had planned to write a letter or a treatise on the great message of salvation, of the love of God in Christ Jesus, on the fundamental doctrines of the Christian faith which are intended for all men. With this intention he was seriously concerned, to it he was giving all diligence. But he found himself obliged to change his plans, to abandon his intention, and that by certain news that had come to him, which caused him to write, instead, of a danger which was threatening the Gospel in the province where his readers were living. Without going into the discussion of the fundamental doctrines at all, Jude here briefly admonishes the Christians not only to be on the defensive, but also on the offensive, to defend themselves and to contend most earnestly and emphatically for the faith, for the Gospel-truth, as it was, once for all, delivered to the saints in the teaching of the apostles. Wherever the apostles had come, they had communicated to men the joyful news of the salvation of all men through the atoning work of Christ. That was the summary and basis of all their teaching. To this truth, therefore, the Christians must cling, for this they must battle with all the power at their command.

The apostle now describes the danger: For by stealth certain men have slipped in, who long ago have been designated beforehand to this condemnation, ungodly, who pervert the grace of our God to lasciviousness and deny our one Master and Lord Jesus Christ. The false teachers to whom Jude has reference made use of the same tactics that have been employed by their successors ever since. They did not proclaim themselves as the murderers of men's souls, as liars and deceivers. They crept into the congregations unawares, they slipped in by stealth. Galatians 2:4; 1 Timothy 1:6; 2 Timothy 2:16; they assumed a pious, a sanctimonious behavior; they affected a great zeal for the Gospel. But, as the apostle says, the judgment which characterizes such men, the condemnation which will strike them, has been predicted in Scriptures long ago. Although professing godliness, they were in reality godless; they perverted the free grace of God into lasciviousness, the lust of the flesh; they turned the liberty of the Gospel into the license which their evil nature craved; they became addicted even to public indecency; and they denied both God, the only Lord, and Jesus Christ the Savior. 2 Peter 2:1; 2 Timothy 3:2; Titus 1:10.