“I know that after my departure savage wolves will come in among you, not sparing the flock; and from among your own selves men will arise, speaking perverse things, to draw away the disciples after them. 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 20:28-31)”
Paul was clearly warning the elders at Ephesus that men bent on their own fame and agenda would come swooping in as soon as he left town to draw them away from the simplicity of the gospel. Paul broke down the gospel into two very simple principles a few verses earlier: “repentance toward God and faith in our Lord Jesus Christ”. Nothing more. Nothing less. This is a freedom bigger than many of us are prepared for. It’s so big, in fact, that in the flesh, it’s very easy to drift away from it. Enter, wolves.
Many had already picked up pieces of the gospel and tried to co-opt it for their agenda, taking grace alone and adding to it works. Some said that to follow Christ one would have to first become a Jew via rites and rituals including circumcision. This would have drawn them closer to men, not God (you’ve got to be one of us before you can be one of His). Paul outright condemns this practice and explains in later letters that it’s grace and grace alone via repentance and faith. Repentance turns us away from sin and faith knits us in to God.