Paul argues that to love your neighbor as yourself, from Leviticus 19:18, is (1) our only debt to one another and (2) is the fulfillment of the law because it sums up the whole law.
It was this text in Romans 13 that convinced me that not only did Jesus teach the Jesus Creed as an adaptation of the Shema (Mark 12:28-32), amending it by adding Leviticus 19:18 to the standard Shema, but expected his followers to repeat it daily — as they had repeated Shema at least twice a day since the days of Deuteronomy 6:4-5.
The question that I asked was not just how often Leviticus 19:18 shows up in Judaism from the days it was given until Jesus (for which there is evidence that it played a significant role), but what role it played for Jesus and for the early Christians. That Paul repeats the Lev 19 part here, and then also at Gal 5, and then that it shows up in James 2 — in a letter that shows tension with Paul, and then everywhere in 1 John convinced me that the followers of Jesus not only incorporated love of others as central but repeated this as a “creed”-like statement on a daily basis. That led to my own practice of saying in my prayers before the Lord’s Prayer. (I have to insert the statement into the prayer book tradition.)
What amazes me every time I think about what Paul says here in Romans 13:8-10 is that he thinks — as does Jesus — that the whole Torah (seen as ethical demand) is summed up in this one commandment: if you love your neighbor, you do everything all those other commandments are getting at. That’s how central love was to Paul.