Doctrine

'Other people may be taken in by the false teacher who deny Christ, but John does not believe that his 'children' will do so. They have their source in God cf 3:10), and consequently they have the inner power of the truth to enable them to withstand error. In this sense they can be said to have 'overcome' the upholders of false teaching. This probably does not mean that hey have physically driven them out of the church; rather they have proved victorious over the temptation to accept false doctrine. False belief is as much a sin as unrighteous behavior or lack of love [emphasis mine]. Victory over it, however, is not due to any innate strength of believers, but rather to the fact that the One who lives in them is greater, i.e., more powerful, than the one at work in the world. God is mightier than the evil one."

Howard Marshall, The Epistles of John, p. 208