4 For there are certain men who crept in secretly, even those who were long ago written about for this condemnation: ungodly men, turning the grace of our God into indecency, and denying our only Master, God, and Lord, Jesus Christ.
“For there are certain men” is in reference to false teachers who sound and look like Christians.
“crept in secretly” It is very hard to see a false teacher coming our way because they blend in very well with the rest of the believers.
“even those who were long ago written about for this condemnation” 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.
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.