MOST PEOPLE WHO HAVE READ 1 John a few times know that John discusses a number of evidences (some commentators call them "tests" or "tests of life") that clarify who truly is a Christian. Most people see three tests: (a) a test of truth, in particular the truth that Jesus is the Son of God; (b) a test of obedience, in particular obedience to the commands of Jesus; (c) a test of love, in particular love for our brothers and sisters. The danger lies in thinking that these "tests" somehow make independent contributions, as if a person might hope to pass two out of three. But toward the end of this epistle, not least in 1 John 5:1-5, these three tests come together in such a way that they are not independent at all. They all hang together.
This paragraph begins with the truth test, with the person "who believes that Jesus is the Christ" (1 John 5:1). That person is born of God---a point repeatedly reiterated in John's writings. But everyone who is born of God will surely love others who have been born of God---spiritual siblings, as it were (1 John 5:1). Thus the truth test is linked, through the new birth, to the love test. How then do we know that we really do love the children of God? Well, first of all, by loving God himself, and then in consequence carrying out his commands (1 John 5:2). Indeed, it is ridiculous to claim to love God and not obey him. So obvious is this that one might go so far as to say that "love for God" is "to obey his commands" (1 John 5:3). Of course, John has already reminded his readers that one of Jesus' central commands, his "new commandment," is that his disciples love one another (1 John 2:3-11; 3:11-20; cf. John 13:34-35). Thus the love test is tied to the obedience test at several levels.
One must not think that Christianity is nothing more than tough-minded obedience. The truth is that Jesus' commands "are not burdensome" (1 John 5:3), for in the new birth God has given us the power to perform what Jesus commands, the ability to overcome "the world" (1 John 5:4-5; cf. 2:15-17). Who, then, has this power to overcome the world? Those who are born again, those who have genuine faith, of course---and genuine faith is defined in terms of faith's object, namely the truth that Jesus truly is the Son of God. Thus the test of obedience, and with it the test of love, is tied back to the truth test.
The glorious reality is that, in the Christian way, truth and ethics are tied together. Creedal confession and transformed living go hand in hand. Any other alternative is either superstition or humbug.