God is love, and those who abide in love abide in God, and God abides in them.I understand from this verse that sin which ruptures our relationship with God, sin which separates us from God so that he no longer abides in us nor do we abide in him, can be understood as a failure to love in the manner that is natural to God. Put another way, to remain in friendship with God means to love in the way that is natural to God, to love in a way that reflects that same love which is God himself. The failure to love in this manner is effectively to cut off friendship with God.
Why is this important? Why does this matter? In this day and age, many people simply do not see why Christians think some things are sins. Some persons talk and think as if God simply named certain things at random and declared them sins, perhaps for no other reason than to rain on our parade. But this verse suggests a different line of thinking altogether: rather than being arbitrary in his declarations of sin, God himself in his love functions as the standard and measure of the life a human should live; sin is a failure to conform with the love that God himself is. God wants us to be like him and to reflect his character; this is what we were made for. Sin consequently is the failure to maintain this likeness and to step outside of the boundaries dictated for human nature.
I want to emphasize this point with respect to sexual sin. Love, of course, is a concern for the good of the other person for her own sake, a respect for her intrinsic dignity and a refusal to objectify her or to treat her merely as a means to personal gain. Sexual sin, however, constitutes a failure to adopt this attitude towards the other person with respect to her sexuality. This is why lust is always a sin: it means gazing upon the other person merely as a sexual object, merely as a means for the satisfaction of sexual desire. (It goes without saying that sexual attraction as such is not the same as lust.)
Sex outside of the context of marriage is also a sin, and a grave one. The desire to engage in sexual activity with the other person without fundamentally committing oneself to her, without accepting responsibility for the natural consequence of sex (namely, procreation), demonstrates a level of objectification and disrespect for the full being of the other person. The same line of reasoning clearly applies to intentionally and unnaturally non-procreative sex within the context of marriage.
I mention all of these things because sexual ethics is one perennially controversial element of Christian teaching, even among Christians themselves who in large numbers have abandoned the traditional views (and not typically for any good reasons). There is something about the unrestrained sexual impulse that presents a deep and fundamental impediment to religious devotion. John of Damascus writes the following about sinful postlapsum humans:
For, since [man] had been created half way between God and matter, should he be freed from his natural relationship to creatures and united to God by keeping the commandment, then he was to be permanently united to God and immutably rooted in good. Should he, on the other hand through his disobedience turn his mind away from his Author—I mean God—and tend rather toward matter, then he was to be associated with corruption, to become passible rather than impassible, and mortal rather than immortal. He was to stand in need of carnal copulation and seminal generation, and because of his attachment to life was not only to cling to these pleasures as if they were necessary to sustain his life, but also to hate without limit such as would think of depriving him of them (De Fide Orthodoxa II:30).The Damascene apparently connects the obsession with sex and the violent resistance to the attempt to limit engagement in sex to some kind of deep awareness of death, guilt, and separation from God. Perhaps Ernest Becker might say the same thing, even if in different terms and for different reasons. In any case, the central point and observation to note here is this: sex and exaggerated attachment to it presents a powerful, sometimes violent impediment to true religion and morals.
But it is not a thinking or rational obstacle; it is unthinking. The arguments in favor of Christian sexual ethics are relatively clear, and the responses to the arguments are in many cases attempts to confuse matters by appeal to alleged persistent ambiguities and uncertainties about issues of metaphysics and nature with relatively easy answers. Other persons are just overwhelmed at the prospect of having to limit themselves in this particular domain of their lives. But that it is difficult, or that it provokes a response of revulsion, does nothing to show that Christian sexual ethics is false.
The truth remains in spite of all this: those who live outside of love, those who knowingly fail to love in the manner which God has taught us through the self-sacrifice of Christ (cf. 1 John 4:7-10), sin in a way that fractures their friendship with God. Precisely because the desires involved in this domain of life are so powerful and often overwhelming, we therefore ought to be increasingly careful and watchful over ourselves. And we should not forget this profound truth from the Damascene: it is impossible to observe the commandments of the Lord except by patience and prayer (De Fide Orthodoxa IV:22).