"For certain men have crept in unnoticed, who long ago were marked out for this condemnation, ungodly men, who turn the grace of our God into lewdness, and deny the only Lord God and our Lord Jesus Christ" (Jude 4).
The phrase "crept in unnoticed" is one compound word in the Greek language, pareiseduesan, used only here in the New Testament. This long word is derived from the words "para" (beside), "eis" (in), and "duo" (to sink or plunge), thus meaning "to slip in secretly as by a side door." "Who long ago were marked out" is another long Greek word, progegrammenoi, which simply means "to write of beforehand." This could refer either to the decree of predestination or to written prophecies like that of Enoch's in verses 14-15. Either way, the point made is that these "ungodly men" are not "unknown" to God, though they have "slipped in" without the notice of men.
Note that these "crept in" false teachers deny Christ's Lordship, not only verbally, but by their lewd, ungodly lives (they profess "grace," but are in reality "graceless"). Here we see the explanation for all the foolish teachings and "damnable heresies" (2 Peter 2:1) that plague our world. Not only is our "creed" (what we believe) important, but also our "conduct" (how we live). "Judgment" and "condemnation" await all false prophets, including all those who "follow their destructive ways" (2 Peter 2:2-3). Judgment may be delayed, but it will eventually fall on all apostates, "because of whom the way of truth will be blasphemed." What a bad name false "Christians" give to Christianity! Because of this we are constantly exhorted in Scripture to "flee" evil things, and "follow" good things (I Timothy 6:1-2; 2 Timothy 2:22). False prophets are thus known by their "doctrine" and their "deportment." But they are indeed "wolves in sheep's clothing" (Matthew 7:15), for they do not announce themselves as wolves or as being sent of Satan.
When we see men and women in the church of our day belittling the doctrine of God's sovereignty, while making great strides to entertain people, is this not a sign of apostates? And if there is continual, lewd, ungodly sensualness in the lives of leaders in the modern church, does this not prove the validity of these warnings from Christ and His apostles? A marked "strong delusion" (2 Thessalonians 2:11) has gripped modern society (religious and political), and it behooves all of us to be constantly examining ourselves to make sure we are not among those who have "crept in unnoticed." And may we always solemnly remember Ecclesiastes 8:10-13 -- that all sinning will some day be over, and in the end it will only "be well with those who fear God."