The first thing you would have asked, if you were an early Christian, is why the angel had a measuring rod.
Ezekiel 40:2-4 –
"In the visions of God, He brought me into the land of Israel, and set me upon a very high mountain: upon which there was as the building of a city, bending towards the south. And he brought me into there.
"And behold! a man whose appearance was like the appearance of brass, with a line of flax in his hand, and a measuring reed in his hand, and he stood in the gate.
"And this man said to me, “Son of man, see with your eyes and hear with your ears, and set your heart upon all that I shall show you. For you are brought hither that these things may be shown to you. Declare all that you see, to the house of Israel.”
Obviously this isn’t a man; he’s an angel. It’s the same angel whom we meet in the Book of Revelation (which is of course full of Ezekiel references).
The angel uses his “measuring reed” or “measuring rod” (translations differ) to measure the Temple. After he has measured the whole Temple, this is what happens:
Ezekiel 43:1-12 –
"And he brought me to the gate that looked towards the east.
“And behold the glory of the God of Israel came in by the way of the east.”
- The East is the Biblical direction of the Messiah’s coming to Jerusalem as well as God’s, and it is the direction of the Second Coming, according to the angels present at Jesus’ Ascension. [In accordance with a lot of psalms, too.]
“And his voice was like the noise of many waters, and the earth shone with his majesty. And I saw the vision according to the appearance which I had seen when he came to destroy the city; and the appearance was according to the vision which I had seen by the river Chobar. And I fell upon my face.”
That probably sounds familiar… from the Book of Revelation…
"And the majesty of the Lord went into the temple by the way of the gate that looked to the east. And the spirit lifted me up and brought me into the inner court: and behold the house was filled with the glory of the Lord.
"And I heard one speaking to me out of the house, and the man that stood by me said to me, "Son of man, the place of my throne, and the place of the soles of my feet, where I dwell in the midst of the children of Israel for ever.
"And the house of Israel shall no more profane my holy name, they and their kings by their fornications, and by the carcasses of their kings, and by the high places. They who have set their threshold by my threshold, and their posts by my posts.
“And there was but a wall between me and them: and they profaned my holy name by the abominations which they committed: for which reason I consumed them in my wrath. Now therefore let them put away their fornications, and the carcasses of their kings far from me; and I will dwell in the midst of them for ever.”"
Like, say, in the New Jerusalem! See, it all ties together!
“But son of man, you show the temple to the house of Israel, and let them be ashamed of their iniquities, and let them measure the building and be ashamed of all that they have done. Show them the form of the house… and all its ordinances, and all its order, and all its laws. And you shall write it in their sight: that they may keep the whole form of it and its ordinances, and do them. This is the law of the house upon the top of the mountain. All its border round about is most holy. This then is the law of the house.”
So that’s why John is being shown the angel measuring the Temple. So that he can tell the Christians the same thing Ezekiel told Israel - to mend their ways and follow God’s laws and the Gospel. Just as the New Jerusalem is perfect, so God’s law is perfect. We’re supposed to try to be perfect too. And if we’re not perfect, we look to Jesus as the perfect offering for sins, just as Ezekiel directs Israel to make sin offerings.
The Book of Revelation says this stuff in a very compressed way, because it expects its hearers to be familiar with all the important prophecies and Biblical books.