Jesus summarised the human aspect of the law as “love your neighbour as yourself” (Matt 5:43; 19:19; 22:39; Mark 12:31; Luke 10:27). Paul (Rom 13:9-10; Gal 5:14) and James (James 2:8) use it similarly – as a summary of the law. Its widespread use (Gospels, Paul, James) indicates it must have been broadly accepted in the early church as a summary of what it meant to act in a godly manner to all people.
The question, of course, is: what does it mean?
I’ve heard it said “well I don’t have to love people more than I love myself – only as myself”. So it becomes a statement of neat reciprocal justice.
But that doesn’t fit with the cross. In the cross, Christ gave himself totally, for undeserving people. You can’t give yourself more completely for someone than dying for them. You can’t get less deserving than sinners who spurn the holy and loving God. The cross has nothing to do with reciprocal justice; it’s complete self-giving love.
Perhaps we’re thinking too individualistically about it. Perhaps it doesn’t mean “treat your neighbour the same as you want to be treated”, but it means “care for others with the same instinctive protection as you would care for yourself.”
When our bodies hurt, we instinctively protect ourselves. When our bodies are in need (hungry, thirsty, hot, cold…) we instinctively nurture ourselves. That’s normal and healthy. If people don’t look after themselves, they have some mental or physical problem, and need external help.
Perhaps Jesus meant we should care for, protect, and nurture the others around us in that same instinctive protection as we normally accord ourselves. So when someone around us is hurt, we automatically protect them. If they’re in need, we give what we have to fill that need.
This interpretation fits with the OT origin of the phrase. It comes from Leviticus 19:18, where it closes off a list of commands about caring for weak people (Lev 19:9-18). It fits with the story of the good Samaritan (Luke 10), who used his own oil and wine, donkey and money to care for someone who should be his enemy. It fits with the context of Rom 13, Gal 5, and the numerous injunctions to practical good in James and 1 John. It's also consistent with what Paul says about the members of the body caring for each other: "If one part suffers, every part suffers with it; if one part is honoured, every part rejoices with it" (1 Cor 12:26).
The only thing with this interpretation is – it’s really inconvenient. Because it means I can’t think of anything that’s ‘mine’ as being purely ‘mine’ for my own benefit without considering ‘you’. And if you are in need, then what’s mine is yours. I'm not renouncing private property - this isn't some sort of communism - it's just that Godliness demands that I dispose of my resources not for myself, but for others. Just like Jesus did. But that can be really inconvenient for someone as greedy & selfish as I am.