None can ascend to heaven without faith; in any case, he who has so ascended there will be cast out wherefore, faith must be zealously preserved. We ourselves each have a heaven within, the gates whereof must be opened and be raised by confession of the Godhead of Christ, which gates are not raised by Arians, nor by those who seek the Son amongst earthly things, and who must therefore, like the Magdalene, be sent back to the apostles, against whom the gates of hell shall not prevail. Scriptures are cited to show that the servant of the Lord must not diminish any of his Master's honour.
15. 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.
16. Suppose, however, that one who is unworthy has crept up, has stolen past the principalities who keep the gates of heaven, has sat down at the supper of the Lord; when the Lord of the banquet enters, and sees one not clad in the wedding garment of the Faith, He will cast him into outer darkness, where is weeping and gnashing of teeth, if he keep not the Faith and peace.
17. Let us, therefore, keep the wedding garment which we have received, and not deny Christ that which is His own, Whose omnipotence angels announce, prophets foretel, apostles witness to, even as we have already shown above.
18. Perchance, indeed, the prophet has spoken of His entering in not only with regard to the gates of the universal heaven; for there be other heavens also whereinto the Word of God passes, whereof it is said: “We have a great Priest, a High Priest, Who has passed through the heavens, Jesus, the Son of God.” What are those heavens, but even the heavens whereof the prophet says that “the heavens declare the glory of God”?
19. For Christ stands at the door of your soul. Hear Him speaking. “Behold, I stand at the door, and knock: if any man open to Me, I will come in to him, and I will sup with him, and he with Me.” And the Church says, speaking of Him: “The voice of my brother sounds at the door.”
20. He stands, then— but not alone, for before Him go angels, saying: “Lift up the gates, O you the princes.” What gates? Even those of the which the Psalmist sings in another place also: “Open to me the gates of righteousness.” Open, then, your gates to Christ, that He may come into you— open the gates of righteousness, the gates of chastity, the gates of courage and wisdom.
21. Believe the message of the angels: “Be lift up, you everlasting doors, and the King of glory shall come in, the Lord of Sabaoth.” Your gate is the loud confession made with faithful voice; it is the door of the Lord, which the Apostle desires to have opened for him, as he says: “That a door of the word may be opened for me, to proclaim the mystery of Christ.”
22. Let your gate, then, be opened to Christ, and let it be not only opened, but lifted up, if, indeed, it be eternal and not condemned to ruin; for it is written: “And be lifted up, you everlasting doors.” The lintel was lift up for Isaiah, when the seraph touched his lips and he saw the Lord of Sabaoth.
23. Your gates shall be lifted up, then, if you believe the Son of God to be eternal, omnipotent, above and beyond all praise and understanding, knowing all things, both past and to come, while if you judge Him to be of limited power and knowledge, and subordinate, you lift not up the everlasting doors.
24. Be your gates lifted up, then, that Christ may come in unto you, not such a Christ as the Arians take Him to be— petty, and weak, and menial— but Christ in the form of God, Christ with the Father; that He may enter such as He is, exalted above the heaven and all things; and that He may send forth upon you His Holy Spirit. It is expedient for you that you should believe that He has ascended and is sitting at the right hand of the Father, for if in impious thought you detain Him among things created and earthly, if He depart not for you, ascend not for you, then to you the Comforter shall not come, even as Christ Himself has told us: “For if I go not away, the Comforter will not come unto you, but if I depart, I will send Him unto you.”
25. But if you should seek Him among earthly beings, even as Mary of Magdala sought Him, take heed lest He say to you, as unto her: “Touch Me not, for I am not yet ascended unto My Father.” For your gates are narrow— they give me no passage— they cannot be lifted up, and therefore I cannot come in.
26. Go your way, therefore, to my brethren— that is, to those everlasting doors, which, as soon as they see Jesus, are lifted up. Peter is an “everlasting door,” against whom the gates of hell shall not prevail. John and James, the sons of thunder, to wit, are “everlasting doom.” Everlasting are the doors of the Church, where the prophet, desirous to proclaim the praises of Christ, says: “That I may tell all your praises in the gates of the daughter of Sion.”
27. Great, therefore, is the mystery of Christ, before which even angels stood amazed and bewildered. For this cause, then, it is your duty to worship Him, and, being a servant, you ought not to detract from your Lord. Ignorance you may not plead, for to this end He came down, that you may believe; if you believe not, He has not come down for you, has not suffered for you. “If I had not come,” says the Scripture, “and spoken with them, they would have no sin: but now have they no excuse for their sin. He that hates Me, hates My Father also.” Who, then, hates Christ, if not he who speaks to His dishonour?— for as it is love's part to render, so it is hate's to withdraw honour. He who hates, calls in question; he who loves, pays reverence.
Source: On the Christian Faith (De fide) (New Advent)