I seem to remember something from high school -- not doctrine, I don't think, but maybe educated speculation(?) -- that the idea of the Incarnation itself was the test. And I guess that, too, was a "hard saying" that not all the angels could accept.
Thus the evil one in Genesis 3 goes after the first woman, Eve, because he had been shown a vision of the Woman with Child and was immediately searching for herIs this the predestined Virgin? Let us tempt her! Let us ruin Gods plan and make ourself the center of creation! Let us entice her to be as gods (Gen. 3:5) apart from Christ! The serpent allured her and she disobeyed Gods plan which was that we might receive of the fullness of the Godhead (cf. Eph. 3:19; Col. 2:8-10; Jn. 1:16) and become partakers of the divine nature (2 Pt. 1:4) and sons of God (cf. Jn. 1:12-13) through the Incarnation.
Interestingly, when Jesus Christ walks this earth the demons recognize Him. I know who Thou art, the Holy One of God, says one demon (Mk. 1:24). Another legion of demons cry out from two possessed men in the country of the Gerasenes, What have we to do with Thee, Son of God? Hast Thou come here to torment us before the time? (Mt. 8:29). And the demons fear that He has come to destroy them (cf. Mk. 1:24; Lk. 5:34). Their recognition of the Incarnate Word and their fear of torment and destruction in His presence indicate that they have foreknowledge of Him and that their damnation and torture is to be subject to the God-Man and His Immaculate Mother whom they eternally rejected before the visible world was even created.