34 Hearing that Jesus had silenced the Sadducees, the Pharisees got together. 35 One of them, an expert in the law, tested him with this question: 36 “Teacher, which is the greatest commandment in the Law?”
37 Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’ 38 This is the first and greatest commandment. 39 And the second is like it: ‘Love your neighbor as yourself.’ 40 All the Law and the Prophets hang on these two commandments.” (Matthew 22:34-40 NIV)
Here Jesus says that the entirety of Scripture (at least up to that point) can be summed up this way: our relationship with God will be reflected in our relationships with other people and vice versa. The most important thing that we can do is to love God. And, this concept is absolutely inseparable from what Jesus identifies as the second most important command: that we should love other people. The is the intersection of the vertical (loving and being loved by God, serving and being nurtured by God, etc.) and the horizontal (loving and being loved by other people, nurturing and being nurtured by others).
The Apostle Paul seems to go even a step further in emphasizing the importance of horizontal aspect of love:
For the entire law is fulfilled in keeping this one command: “Love your neighbor as yourself.” (Galatians 5:14)
Evangelical Christians often emphasize the message of Grace and the importance of faith (and faith alone) as a means of salvation. Passages such as Romans 10:9-10 and Ephesians 2:8-9 are oft cited in evangelical circles. But, the notion of "salvation by grace, through faith" encapsulates only the vertical aspect of relationship. We are saved, according to Paul, because God gives us grace. (Vertical relationship from God to us -- top down) We receive God's grace by having faith in Christ. (vertical relationship from us to God -- bottom up)
But, Christ Himself is the intersection between the vertical and the horizontal He is God come to be with His people. But, He is also the Son of Man. And, we cannot truly grasp the vertical relationship of grace and faith without also embracing the horizontal relationship of mutual love and service towards one another.