have overcome [conquered] them definition
have overcome [conquered] them. . means they have rejected the heretical teaching of the anit-christs (i.e. the secessionists). "the one who is in you" refers to the Holy Spirit. "the one who is in the world" is the spirit of antichrist, the devil, who is active in the inhabited earth. The spiritual security of believers rests ultimately upon the Spirit's work within them, and there is no power greater than the Spirit of God that can destroy his work. This is a firm basis of Christian assurance.