4 For some people, who were designated for this judgment long ago, have come in by stealth; they are ungodly, turning the grace of our God into sensuality and denying Jesus Christ, our only Master and Lord.
“For some people” is in reference to false teachers who sound and look like Christians.
“designated for this judgment long ago” There is a specific place in hell assigned for false teachers. The reason is that they purposely try to make Christians if possible abandon their faith in Christ and stop others from getting saved.
“come in by stealth” It is very hard to see a false teacher coming our way because they blend in very well with the rest of the believers.
False teachers do not even have an ounce of godliness, their soul filled with pure darkness. One of their tactics is to make Christians accept all types of sexual sin as not being sinful. Through their teachings and behavior, it shows their rejection of Jesus Christ.