Capstone, cornerstone and keystone

The stone which the builders rejected has become the chief cornerstone.
(Psalms 118,22)

I would like to give a short explanation of the "capstone" which is mentioned in the first strophe:
"After that cry
for the capstone, for the only one
that would keep your walls firm."

At first glance one would never tell how comes the image of capstone into view in this case. To understand its important role among the symbols of this poem, we have to understand what is a capstone. A capstone or a cornerstone is an important element of buildings: this is the first stone, the first element they lay down when they begin to build a building. Laying this cornerstone always happens in ceremonial circumstances, showing: it is an important moment.

Also the cornerstone had a special significance in Israel back then, in the Biblical times: the way this stone was carved and laboured defined the how they built the building, the house or the temple afterwards. It was a huge and heavy stone, for example they used a prismatic, beautiful chalk as a cornerstone at building the Temple at Jerusalem. But the format of the cornerstone could be anything: square, cube, oblong, conical, etc. If they once laid the cornerstone they built and adapted the building exactly to its cornerstone.

The psalm verse I've mentioned above is from an important Psalm regarding the wait for the Messiah, as we can see it in Parabel of the Wicked Vinedressers (Matthew 21,33-46). Jesus Christ quotes Psalm 118: Jesus said to them: "Have you never read in the Scriptues: 'The stone which the builders rejected become the chief cornerstone. This was the Lord's doing and it was marvelous in our eyes.'? Therefore I say to you, the kingdom of God will be taken from you and given to a nation bearing the fruits of it. And whoever falls on this stone will be broken; but on whomever it falls, it will grind him to powder. (Matthew 21,42-43)
Jesus Christ is the cornerstone of the Christianity, and also the Peter in his first letter says: you also, as living stones, are being built up a spiritual house, a holy priesthood, to offer up spiritual sacrifices acceptable to God through Jesus Christ. (1Peter 2,5). He is the fundament on which our salvation has been built and based; and at the same time he's also the keystone (the last stone they place when they finish to build any kind of building).
Also worths to mention the passage by Zechariah: Who are you, O great mountain? Before Zerubbabel you shall become a plain! And he shall bring forth the capstone with shouts of “Grace, grace to it! (Zechariah 4,7), in which he refers the Messiah, therefore Jesus Christ.

If the strophe would stand by itself, we could say that the image of the capstone refers to Jesus Christ, and Zion could be the Church, but I have a hunch that the capstone is rather a symbol for something that is as firm as the fundament of Christianity.

The fact that Alexander demands to cry for the capstone "that would keep your walls firm" indicates a sensitive state of mind or emotional state and - of course - some sort of danger. He also names the reason why the capstone weakened, he says: The wind took with pincers its solidity.

The question is: what does the wind (as symbol) stands for, and what kind of danger awaits Zion?