False prophets had shaken the faith of the Christians that had received this letter.  But the Apostle John reminds them that they had every confidence.  God had displayed the love He had for them by sending His Son (i.e., God's love for us).  He also continues to love us by sending us the Spirit, who abides with us (God's love within us).  By recognizing God's self-giving love toward us sinners, we become joyful and grateful and, in turn, give ourselves to one another in Christian love.  By first having confidence in God's love for us and in us, His love is "perfected" - working through us.