Jesus was saying that He and the Father are of one nature and essence.
Christians claim that this statement proves that Jesus is one or united with God and, consequently, Jesus is God. However, when Jesus died, he did not give up the Father, he only gave up the ghost, "And Jesus cried with a loud voice, and gave up the ghost" (Mark 15:37).
This claimed unity was not available to Jesus when he died; what happened to this unity and why did not the Father die when Jesus died, if Jesus and the Father are one?