There are several biblical indications that it is okay for the Church’s institutions to be magnificent and richly decorated. A major indication to this is the Temple in Jerusalem, which is a prefiguring of the Church. It was vast, magnificent, and richly decorated in at least as many ways as our cathedrals are. In the Bible King David tells us how much he had prepared for it: With great pains I have provided for the house of the LORD a hundred thousand talents of gold, a million talents of silver, and bronze and iron beyond weighing, for there is so much of it; timber and stone too I have provided. To these you must add. (1 Chronicles 22:14) And Solomon did add: So now send me a man skilled to work in gold, silver, bronze, and iron, and in purple, crimson, and blue fabrics, trained also in engraving, to be with the skilled workers who are with me in Judah and Jerusalem, whom David my father provided. Send me also cedar, cypress, and algum timber from Lebanon, for I know that your servants know how to cut timber in Lebanon. And my servants will be with your servants, to prepare timber for me in abundance, for the house I am to build will be great and wonderful. (2 Chronicles 2:7-9) And here is the beginning part of the description of the temple: These are Solomon’s measurements for building the house of God: the length, in cubits of the old standard, was sixty cubits, and the breadth twenty cubits. The vestibule in front of the nave of the house was twenty cubits long, equal to the width of the house; and its height was a hundred and twenty cubits. He overlaid it on the inside with pure gold. The nave he lined with cypress, and covered it with fine gold, and made palms and chains on it. He adorned the house with settings of precious stones. The gold was gold of Parva’im. So he lined the house with gold – its beams, its thresholds, its walls, and its doors; and he carved cherubim on the walls. (2 Chronicles 3:3-7) It was pretty magnificent. The description goes on like that, in more and more detail, to the end of chapter 4, and after that it starts describing all the costly cups and bowls and oils that Solomon brought in for liturgy. So we have more than enough precedent to have all these things in our churches.
Another important consideration is this prophesy, which describes the New Covenant: Your divine throne endures for ever and ever. Your royal scepter is a scepter of equity; you love righteousness and hate wickedness. Therefore God, your God, has anointed you with the oil of gladness above your fellows; your robes are all fragrant with myrrh and aloes and cassia. From ivory palaces stringed instruments make you glad; daughters of kings are among your ladies of honor; at your right hand stands the queen in gold of Ophir. Hear, O daughter, consider, and incline your ear; forget your people and your father’s house; and the king will desire your beauty. Since he is your lord, bow to him; the people of Tyre will sue your favor with gifts, the richest of the people with all kinds of wealth. (Psalms 45:6-13) That shows us that the New Covenant will have magnificent buildings (“ivory palaces”) made with vast wealth from the nations (“the richest of the people [will offer] all kinds of wealth.”) This prophesy is fulfilled in the New Covenant.
I’m sure there are more passages that offer evidence of the magnificent beauty of our cathedrals. It really makes sense when you consider what they’re made for: to be vessels of the presence of God in the Eucharist. If the Old Covenant found enough justification for large and splendid buildings because they bore the presence of God in the Ark, then how much more should we! But as I say, there are more passages that support this, but this post is already getting long. I hope what I’ve posted helps and shows you that we don’t do this out of any contempt for the poor, but out of honor to God and His presence. God bless!