The doorkeepers of the [heavenly] kingdom are careful and they do not play games. They see the soul bearing the marks of her banishment…Then the miserable soul, accusing herself severely of her own thoughtlessness, and howling and wailing and lamenting, remains in that sullen place, cast away as if in a corner, while the incessant and inconsolable wailing takes vengeance forever. (Against Those Who Resent Correction. Migne PG 46: col. 312)
St. Ambrose of Milan ca. 338-397
And therefore [the angels] descrying the approach of the Lord of all, first and only Vanquisher of Death, bade their princes that the gates should be lifted up, saying inadoration, Lift up the gates, such as are princes among you, and be lifted up, O everlasting doors, and the King of glory shall come in.
Yet there were still, even among the hosts of heaven, some that were amazed, overcome with astonishment at such pomp and glory as they had never yet beheld, and therefore they asked: Who is the King of glory? Howbeit, seeing that the angels (as well as ourselves) acquire theirknowledge step by step, and are capable of advancement, they certainly must display differences of power and understanding, for God alone is above and beyond the limits imposed by gradual advance, possessing, as He does, every perfection from everlasting.
Others, again—those, to wit, who had been present at His rising again, those who had seen or who already recognized Him—made reply: It is the Lord, strong and mighty, the Lord mighty in battle. Then, again, sang the multitude of angels, in triumphal chorus: Lift up the gates, O you that are their princes, and be lifted up, you everlasting doors, and the King of glory shall come in. And back again came the challenge of them that stood astonished: Who is that King of glory? For we saw Him having neither form nor comelines; Isa. 53:2 if then it be not He, who is that King of glory?
Whereto answer they which know: The Lord of Hosts, He is the King of glory. Therefore, the Lord of Hosts, He is the Son. How then do the Arians call Him fallible, Whom we believe to be Lord of Hosts, even as we believe of the Father?
What shall we do, then? How shall we ascend unto heaven? There, powers are stationed, principalities drawn up in order, who keep the doors of heaven, and challenge him who ascends. Who shall give me passage, unless I proclaim that Christ is Almighty? The gates are shut—they are not opened to any and every one; not every one who will shall enter, unless he also believes according to the true Faith. The Sovereign’s court is kept under guard. (De Fide Bk 4.9-15)
St. Symeon the New Theologian ca. 949-1022
Those who keep the gates of the Kingdom of Heaven, if they do not see in a Christian the likeness of Christ, as a son to his father, will by no means open to him and allow him to enter. (Homily 2.3-4 The Blessed State)
St. Nicholas Cabasilas ca. 1323-1391
Thus he comes forth, and when he reaches the temple and comes near to it he stands before the closed doors and commands those who stand within the doors to open them for the King of Glory (Ps. 24:7,9), as he utters the very words of David. When he has heard from those who are within the words which David represents the angels as saying to each other when the Saviour ascends into heaven, and when the doors are flung open, he enters the temple with the veiled vessel upon his head. (The Life in Christ, Fifith Book: 2. The Ceremonies of Consecration)