The chariots of God are twenty thousand, even thousands of angels: the Lord is among them, as in Sinai, in the holy place.
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Augustine of Hippo
21. Thus cometh to pass that also which followeth: "The Chariot of God is of ten thousands manifold:" or "of tens of thousands manifold:" or, "ten times thousand times manifold" (ver. 17). For one Greek word, which hath there been used, murioplasion, each Latin interpreter hath rendered as best he could, but in Latin it could not be adequately expressed: for a thousand with the Greeks is called xilia, but muriadejare a number of tens of thousands: for one muriaj are ten thousands. Thus a vast number of saints and believers, who by bearing God become in a manner the chariot of God, he hath signified under this name. By abiding in and guiding this, He conducteth it, as though it were His Chariot, unto the end, as if unto some appointed place. For, "the beginning is Christ; secondly, that are of Christ, at the appearing of Him; then the end." This is Holy Church: which is that which followeth, "thousands of men rejoicing." For in hope they are joyful, until they be conducted unto the end, which now they look for through patience. For admirably, when he had said, "Thousands of men rejoicing:" immediately he added, "The Lord is in them." That we may not wonder why they rejoice, "The Lord is in them." For through many tribulations we must needs enter into the kingdom of God, but, "The Lord is in them." Therefore even if they are as it were sorrowful, yet alway rejoicing, though not now in that same end, to which they have not yet come, yet in hope they are rejoicing, and in tribulation patient: for, "The Lord is in them, in Sina in the holy place." In the interpretations of Hebrew names, we find Sina interpreted commandment: and some other interpretations it has, but I think this to be more agreeable to the present passage. For giving a reason why those thousands rejoice, whereof the Chariot of God doth consist, "The Lord," he saith, "is in them, in Sins in the holy place:" that is, the Lord is in them, in the commandment; which commandment is holy, as saith the Apostle: "Therefore the law indeed is holy, and the commandment is holy, and just, and good." ...