Since the children have flesh and blood, he too shared in their humanity so that by his death he might break the power of him who holds the power of death—that is, the devil
Some notes from a message by Douglas F. Kelly of RTS on the topic of the hypostatic union. I don't always agree with everything Dr. Kelly speaks about (particularly on infant baptising) but these notes were particularly good.
The manhood, or humanity, of Christ, was and is necessary for salvation. Hebrews speaks about how it behooved him to become flesh. If a view of docetism is held, then he is still far away from us.
When we die, what will we offer God? The answer: Jesus' obedience. Jesus is who God most essentially is. He is the character of God. If the diety of Christ is denied, then the foundation of the Christian faith is undermined.