Before Eden, before Adam, before sin the Godhead had determined to create beings who were to be adopted into God's family, and that adoption would be through Jesus. Since the created and temporal cannot enter the realm of the uncreated an incarnation was necessary, God becoming flesh in Christ, truly God, truly man, who as the first fruit and our elder brother incorporated our humanity into Himself, so that, in another development in his line of thought Paul wrote
Eph 2:4-10 But because of his great love for us, God, who is rich in mercy, made us alive with Christ even when we were dead in transgressions it is by grace you have been saved. And God raised us up with Christ and seated us with him in the heavenly realms in Christ Jesus, in order that in the coming ages he might show the incomparable riches of his grace, expressed in his kindness to us in Christ Jesus. For it is by grace you have been saved, through faith and this not from yourselves, it is the gift of God not by works, so that no one can boast. For we are God's workmanship, created in Christ Jesus to do good works, which God prepared in advance for us to do. NIV
We are raised with Christ and seated in heaven. Since you and I are currently still on earth 'being seated in the heavenly realms' must mean that we too are incorporated into Christ's resurrection and life now, with it's complete fulfillment to come when Jesus returns. Flesh and blood cannot enter heaven. Only was we become adopted into God's family through Jesus can we enter the heavenly realms. Thus, the incarnation was always necessary.