In Mark 12:28, a scribe asks Jesus what the greatest commandment is. Jesus responds that the first and greatest commandment is to love God with all your heart, mind, soul, and strength. He then says that the second is like the first, "You shall love your neighbor as yourself." Jesus concludes by saying that there is no commandment greater than these.
The scribe asked Jesus for the most important commandment, and Jesus gave him two. Was Jesus saying that love for God is the greatest commandment while love for neighbor is second in importance? Or was he saying that both love of God and love of neighbor are equally important and rank as the greatest commandments? I'm not sure, but I do know that Jesus wanted to emphasize both.
Why? Because people would rather keep only one of them. In Jesus' day, people wanted to love God but not their neighbors. In Mark 7:10-13, Jesus criticizes the Pharisees because they used corban (a dedication to God) to avoid having to honor their parents. Many scholars will argue that Mark is only Christian propaganda and that the Pharisees were not that bad. I realize that rabbinic literature emphasizes the love of neighbor, but there are always hypocrites. None of us lives up to what we profess to believe.
And there are many people who would prefer to love God rather than their neighbors. First century Jews were very religious, and yet even the Babylonian Talmud says that Jerusalem was destroyed because of Jewish infighting. The New Testament criticizes people in the church for wanting to honor God but not their neighbors. There was strife, pride, and contempt for the poor in Corinth. I John 4:20 states, "If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?" And James points out the utter absurdity of praising God while cursing someone who is made in God's image (James 3:9). Look around you today! The problem still exists. There are wars in the name of religion, and we all know religious people who can be downright hateful.
Today, however, there are many people who want to love their neighbors and not God. Atheists contend that belief in God is unnecessary for a moral life, and you even have some religious people who go along with that sentiment. On some level, that issue existed also in Jesus' day, since there were seemingly moral people who did not worship the God of Israel. Maybe that's one reason that Jesus wanted to emphasize the love of God. That, and the fact that the love of God actually is the most important commandment of all. As Jonathan Edwards pointed out, God has more value, glory, dignity, and righteousness than all of us put together. He deserves the most love.
I'm not going to wrap my brain around the issue of foundationalism--Is belief in God the only possible basis for morality? I'll save that question for another day. All I want to say is that, for Jesus, love of God and love of neighbor are both important.