God began His work ofin the Age of Grace, after the end of the Age of Law. The first incarnation redeemed man from sin through the flesh of Jesus Christ. He saved man from the cross, but man's satanic dispositions lingered on. In the last of days, God judges to purify mankind. It's only when this is done will He end His work of salvation and enter into rest.
He lives among man, experiencing his suffering, and gifts to man His word. All man can touch is God's incarnate flesh. Through Him man can receive salvation and understand all words and truths. The second incarnation, enough to purify man, thus completing all the work and the meaning of His incarnation. The work of God in the flesh will now come to an end. He won't incarnate in flesh.
After this incarnation, His work in flesh and salvation will cease. For He'll have divided man and gained His chosen people. The second incarnation will deliver the forgiven ones. Dispositions will change and they'll be clean. Breaking free from Satan's influence, they'll return to God's throne. Yes, it's the only way to be fully sanctified.
from "The Mystery of the Incarnation (4)" in The Word Appears in the Flesh