So what exactly are the rules for Christian living? Is the bible a 'moral authority'?

James J. Tissot - Pharisees ask Christ
about the Greatest Commandment

"`Teacher, which [is] the great command in the Law?' And Jesus said to him, `Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thine understanding -- this is a first and great command; and the second [is] like to it, Thou shalt love thy neighbor as thyself; on these -- the two commands -- all the law and the prophets do hang.'" (Matt 22:36-40, YLT)

I've stumbled into some discussions lately about Christian ethics (so-called). It's the usual 'yes, love, but ...' thing. For some reason some Christians want all kinds of rules, besides the commandment to love others. The bible, these people say, is a 'moral authority'. Hence they sometimes reiterate Matt 22:36-40, where Jesus explains a Jewish Pharisee that the great(est) commandments in the Law is to love God and your neighbor. But in order to know what it means to love God, the argument goes, we need to know all kinds of things from the old and new testaments. Such as: If God or some biblical author doesn't seem to like homosexuality at certain instances we should probably have a general rule against that.

You would think that 'do unto others as you would want others do to you' (the Golden Rule) is enough, since Jesus calls it the foundation of the whole law in the Sermon on the Plain (Luk 6:31). Not so, apparently.

Hence in a Platonic manner Augustine explained that the Golden Rule is not enough to guide Christian ethics, since this makes it okay to, e.g., swap your wifes, if everyone is ok with it (On the Free Will). Sin is, says Augustine, to love (in the sense of desiring) something that can be lost against one's will. Neighborly love, says Augustine, means helping others to love (desire) God. Pleasure is sin, and will be punished, unless allowed by God (e.g. eating moderately and sexual stuff inside marriage). Such thinking, which more or less identifies Christian love (agape) with desire (eros), has led to all kinds of puritanism throughout Christian history.

But loving God (in the agape sense) means to do what God wills (1 Jn 5:3). It doesn't mean to 'desire God' (as in Platonic eroticism). The Mosaic Law is a guide to what it means to do the will of God. But it turns out that no one is able to do that, and so to love God, except for Jesus of course, who is the end and fulfillment of the Law (Rom 10:4). To have faith in Jesus, therefore, means not to be under the Mosaic Law anymore. Jesus is taking care of that. He didn't come to abolish the Law, of course (Matt 5:17): It still functions as a schoolmaster (Gal 3:24), teaching us that no law can bring us anywhere with God. But we are not under the Law, if we are under grace through faith in Jesus.

But Jesus has given us a new commandment, which is, in fact, a repetition of the second part of the two great commandments in Matt 22:36-40: That of loving our neighbor. What God wills now is neighborly love. For a Christian, to love God is to love one's neighbor.

This is why the law for Christians in the rest of the New Testament (in distinction from Matt 22:36-40, where Jesus is talking to a Jew, not a Christian) is (usually) defined simply as neighborly love:

"All things therefore whatsoever ye would that men should do unto you, even so do ye also unto them: for this is the law and the prophets." (Matt 7:12, ASV)

"Owe no man anything, save to love one another: for he that loveth his neighbor hath fulfilled the law." (Rom 13:8, ASV) 

"For the whole law is fulfilled in one word, even in this: Thou shalt love thy neighbor as thyself." (Gal 5:14) 

"Howbeit if ye fulfil the royal law, according to the scripture, Thou shalt love thy neighbor as thyself, ye do well" (James 2:8)

It's not that complicated.

The whole part (table) of the Law that has to do with loving God is out of the picture now. This does not mean, of course, that we should not love God. How can we do otherwise, when Christ lives in us (Rom 8:10)? (this is why Paul often sounds quite moralistic: don't do this, don't do that). But the only commandment is to love others, which means, that we don't need to speculate about what rules to follow in order to love God. We are not under that law anymore.

Whatever God originally intended for this or that thing (e.g. marriage), there is now just this one commandment: Love thy neighbor as thyself.

Moreover: Love is not something that can be put into clear definitions. If we have not experienced it, we have no idea what it is. We love him, because he loved us first (1 Jn 4:19).

It is not that love is completely incomprehensible or cannot be defined at all. But such definitions only help to point indirectly at something we already know, intuitively. This is why, when explaining how love looks like, Paul uses mostly negative definitions: It does not envy, it does not boast, it is not proud, etc. (1 Cor 23).

Hence the bible is not a 'moral authority', but only a means to put words on and help us understand something that we already know, a guidance at max (but of course at times a very good and useful one). But if we do not already know, neither will we understand the meaning of the bible: Jesus Christ, the Word of God Himself.