Of course, it is impossible to locate one central or "most important" verse or passage in the scriptures. Central to whom? Most important for whom? There are, however, a few passages upon which all else seems to depend, not just in the abstract, but in terms of practical theology. John 1:14 is one of these passages, for it is the clearest, most succinct, descriptions of God become flesh, or the Incarnation of the Word (Jesus Christ).
This passage is so central, in fact, that without it (and/or perhaps Colossians 1:15-20), I am not sure I would be drawn to Christianity. It is the Incarnation, the very fact that Jesus Christ became flesh and dwelt among humanity, that makes Christianity unique among world religions, and without which Christianity makes very little sense. John 1:14-18 is so well-known that Christians often read through it without catching its significance, both in terms of personal transformation and as a model of living the Christian life.
First, "the Word became flesh." This indicates something radical about both the Word and flesh (i.e. the material human being). About the Word, it indicates that God, or Spirit, is not in conflict with the material world. Against the Gnostics and much Greek philosophy contemporary to John's time, the passage completely opposes the view that only spirit is good, and that matter is bad - a popular view of the time, and a view which continues among too many Christians today. The body is good. Food is good. Pleasure is good. The flesh is good. The Word was not polluted by becoming flesh. By partaking in embodiment, the Word called flesh, "blessed."
Second, the flesh - or embodiment - appears to be that vehicle by which humans can be fully known by the divine, and vice versa. God is Spirit, and only in becoming flesh in Christ Jesus was the divine able to empathize with our weaknesses (Hebrews 4:15) and fully know His creation as He knows Himself. It is hardly insignificant that Jesus maintains a physical, resurrected body (Philippians 3:20-21). His incarnation was not a 33-year costume he donned just for appearances, but the full revelation of divinity. This is the foundation of orthodox Christian theology.
What does this mean for us? Well, if we claim to be followers of Jesus, and if we maintain that Jesus is the fullest revelation of the divine, it means that stepping into another's reality is the model of perfect Love. The Word (Logos: ultimate reality) entered into our reality in order to know us (and love us) fully and unconditionally. This models, for us, how to know and love others fully and unconditionally. Just as the Word stepped into our reality, we are called to step into the reality of those unlike us. Therein lies the framework for mutual relationality.