The Great Commandment exists in three forms==

1. Listed as separate commandments. The first is listed in Deuteronomy 6:5 (“You shall love the Lord your God with all your heart, and with all your soul, and with all your might.”), while the second part is in Leviticus 19:18 (“you shall love your neighbor as yourself”).
2. Listed as one inseparable commandment in Luke 10:27 (“You shall love the Lord your God with all your heart, and with all your soul, and with all your strength, and with all your mind; and your neighbor as yourself.”)
3. Listed as two separate commandments but treated as if they are one. This is done in Matthew 22:35- and in Mark 12:28-.

All three forms have advantages. It is useful to see them as separate since they have separate objects and separate relationships. One relates to one’s relationship with God and the other relates to relationships with other people. Treating it as one rule makes it clear that one cannot be claim to obey the Great Commandment if one is obeying only part of it.

But there is value in seeing them as separate but linked. Jesus did this in Matthew 22 and Mark 12. In Matthew, Jesus was asked which commandment is “first.” Jesus answered it but gave the first two rather than simply the first. In Mark, Jesus was asked which commandment is “greatest.” Jesus answered it but then mentions the second, and then says that in these two laws there is no commandment greater.

I believe the link is important. Recall the quote by Blaise Pascal

“Men never do evil so completely and cheerfully as when they do it from religious conviction.”

-Blaise Pascal, Pensees

Apparently this is a bad translation of Pascal’s quote (“Jamais on ne fait le mal ſi pleinement & ſi gayement, que quand on le fait par un faux principe de conſcience”)— but I think there is an underlying truth in it anyway.

PEOPLE JOYFUL DO EVIL WHEN THEY BELIEVE THAT THEY ARE DOING IT AT GOD’S REQUEST AND WITH HIS BLESSING.

Linking the two parts of the Great Commandment undermines this in two ways:

#1. Most obviously, doing evil against people violates the second part of the commandment.

#2. More subtly, the second part of commandment points to the nature of its author. If we are to love God, we have to know who that God is. That God is the God who loves all people and expects us to do likewise.

It is easy to think of these as separate. It is easy to think of oneself as loving God. And then it is easy to think that one should hate what God hates. We are pretty sure God hates sin, so it seems pretty reasonable to assume that we show God how much we love Him by showing how much we hate those sins God hates. While we may mouth the principle, “Hate the sin, love the sinner,” it is really hard to demonstrate the former while simultaneously demonstrating the latter. (It is curious that did not appear to be traumatized by living in a sinful world, and appeared to make no real effort to convince His Father of His hate of sin.)

The previous paragraph doesn’t work. What works is recognizing that the God we are to love is the author of both parts of the Great Commandment.