If your fundamental is a man dying on the cross for his enemies, if the very heart of your self-image and your religion is a man praying for his enemies as he died for them, sacrificing for them, loving them - if that sinks into your heart of hearts, it's going to produce the kind of life that the early Christians produced. The most inclusive possible life out of the most exclusive possible claim - and that is this is the truth. But what is the truth? The truth is a God become weak, loving and dying for the people who opposed him, dying forgiving them.
In Ephesians 5, Paul shows us that even on earth Jesus did not use his power to oppress us but sacrificed everything to bring us into union with him. And this takes us beyond the philosophical to the personal and the practical. If God had the gospel of Jesus's salvation in mind when he established marriage, then marriage only 'works' to the degree that approximates the pattern of God's self-giving love in Christ.
We instinctively tend to limit for whom we exert ourselves. We do it for people like us, and for people whom we like. Jesus will have none of that. By depicting a Samaritan helping a Jew, Jesus could not have found a more forceful way to say that anyone at all in need - regardless of race, politics, class, and religion - is your neighbour. Not everyone is your brother or sister in faith, but everyone is your neighbour, and you must love your neighbour.