Everyone who believes that Jesus is the Christ has been born of God, and everyone who loves the Father also loves the one born of him. This is how we know that we love God’s children: when we love God and obey his commands.
1 John 5:1-2, CSB
When Jesus was asked what the most important of God’s laws are, he said that the two most important laws were to love God and to love one’s neighbour (Mark 12:38-31). The fact that he mentions these together in this way is no accident, for these two laws put together summarize the whole of God’s moral law, especially as found in the Ten Commandments, with the first four commandments teaching us how to love God, and the latter six commandments teaching us how to love our neighbours.
Furthermore, these two greatest commandments are closely connected in such a way that one cannot properly fulfill one without the other, and vice versa. This connects with the above-mentioned verse from St. John, who states that everyone who loves God also loves his brethren. Theologian and counsellor Jay E. Adams said it best when he wrote: “The sum of the commandments is love toward God and toward one’s neighbour. By linking those two great commands, Jesus made it clear that we must fully consider both dimensions. John, in his first letter, likewise demonstrated how you can’t have the one without the other.”
Having good relationships with our neighbours is such that without it, our relationship with God suffers as well. To again quote Adams, “unresolved bad relationships with each other precludes good relationships with God.” The reason for this is that as Christians, we are built together into one body, and if one member of the body suffers, the whole body suffers, and since Christians collectively make up the temple of the Holy Spirit, one cannot allow the body to suffer without grieving the Spirit. (1 Corinthians 6:19, 12:26)
Hence, Christians must strive to love one another and to pursue harmonious relationships as much as possible. As Christ commanded: “Love one another. Just as I have loved you, you are also to love one another. By this everyone will know that you are my disciples, if you love one another.” (John 13:34-35)
 Jay E. Adams, How to Help People Change: The Four-Step Biblical Process (Zondervan, 1986), 4.