You are right when you say Christians believe that Jesus was the unique Son of God. In fact, they have held this belief for almost 2,000 years—since the very beginning of the Christian faith.
Why is this? The main reason is because this is the consistent testimony of the Bible. Jesus Himself declared that He was both fully man and fully God. To those who opposed Him, He said, "I and the Father are one" (John 10:30). They understood exactly what He meant, and even started to stone Him "because you, a mere man, claim to be God" (John 10:33).
This also was the consistent testimony of the earliest Christians (including His closest disciples). John declared, "In the beginning was the Word, and the Word was with God, and the Word was God. ... We have seen his glory, the glory of the One and Only, who came from the Father" (John 1:1,14).
Why is the divinity of Jesus so important? Because our greatest need is to have our sins forgiven, and only a divine Savior—and therefore without sin—could become the final sacrifice for our sins. I invite you to see Jesus as He truly was: God in human flesh, coming from heaven to save us from our sins. Then turn to Him in repentance and faith, and give your life to Him today.