If there was any doubt amongst Jesus' followers as to His true identity, then His resurrection form the dead showed clearly that He was God's Son. This is one of the very important reasons why the bodily Resurrection of Christ is such an important truth in Christianity. We preach about a Savior who is ALIVE! We don't have a "Founder of Our Religion" laying in a grave someplace. He is not someone who only lived a good life and left behind a nice legacy. We don't preach about someone who said nice things and will be remembered for such practical lessons like the "Golden Rule." NO!
We preach Jesus Christ, crucified, buried, and RISEN AGAIN!! We preach a living Savior who is none other than God's Son!" And God wanted everyone to know that!
Remember, according to Mark 15:39, the centurion on duty at the scene of Christ's crucifixion and a witness to what Jesus said on the cross, declared, "This man truly was the Son of God!" After Christ's resurrection, God said the same thing! "This Man truly IS the Son of God!"