I have a friend who claims Jesus wasn't the unique Son of God (as I've always believed), but that He was only one of many sons of God, and in reality we are all sons (and daughters) of God. Is she right? She's very sincere.
I’m sure your friend is sincere–but this is not what the Bible teaches about Jesus. Nor is it what Christians have believed (and been willing to die for) across the centuries.
The Gospels make it clear that Jesus was absolutely unique–and the reason is because He was the unique Son of God, sent from heaven to save us from our sins. Yes, He was fully human–but He was also fully God. Centuries before He was born, the prophets predicted that “they will call him Immanuel–which means, ‘God with us'” (Matthew 1:23). Jesus Himself declared, “I and the Father are one” (John 10:30). He also said, “Anyone who has seen me has seen the Father” (John 14:9).
How do we know these claims are true? We know it first of all because of Jesus’ character. His enemies constantly looked for something wrong in His life–but without success, for He alone was without sin. We know it also because of His power. He alone could cause a storm to cease or a dead child to come back to life–powers only God could have. We know it especially because of His resurrection, by which He conquered death and hell.
Yes, Jesus was the unique Son of God–but why is it important? The reason is because only a divine Saviour could save us from our sins–and He has, through His death on the cross for us. Pray for your friend, that she will realize what Jesus has done for her, and then turn in faith and commit her life to Him.