And in the middle of the plain he showed me a large white rock that had arisen out of the plain. And the rock was more lofty than the mountains, rectangular in shape, so as to be capable of containing the whole world: and that rock was old, having a gate cut out of it; and the cutting out of the gate seemed to me as if recently done. And the gate glittered to such a degree under the sunbeams, that I marvelled at the splendour of the gate; and round about the gate were standing twelve virgins.
The four who stood at the corners seemed to me more distinguished than the others— they were all, however, distinguished— and they were standing at the four parts of the gate; two virgins between each part. And they were clothed with linen tunics, and gracefully girded, having their right shoulders exposed, as if about to bear some burden. Thus they stood ready; for they were exceedingly cheerful and eager. After I had seen these things, I marvelled in myself, because I was beholding great and glorious sights.
And again I was perplexed about the virgins, because, although so delicate, they were standing courageously, as if about to carry the whole heavens. And the Shepherd said to me “Why are you reasoning in yourself, and perplexing your mind, and distressing yourself? For the things which you cannot understand, do not attempt to comprehend, as if you were wise; but ask the Lord, that you may receive understanding and know them. You cannot see what is behind you, but you see what is before. Whatever, then, you cannot see, let alone, and do not torment yourself about it: but what you see, make yourself master of it, and do not waste your labour about other things; and I will explain to you everything that I show you. Look therefore, on the things that remain.”
Source: The Pastor (or "The Shepherd") (New Advent)