When Christ says that the Son has all the Father has, it is not a generic statement. It's context is thus:
Howbeit when he, the Spirit of truth, is come, he shall guide you into all the truth: for he shall not speak from himself; but what things soever he shall hear, [these] shall he speak: and he shall declare unto you the things that are to come.
He shall glorify me: for he shall take of mine, and shall declare [it] unto you.
All things whatsoever the Father hath are mine: therefore said I, that he taketh of mine, and shall declare [it] unto you.
St. John 16:7-15
The "all" Christ is referring to is the Glory of God. "All things..." is an explanation of why "He shall glorify me...". And to make it not dubious, the Apostle even wrote "therefore said I" and then mentioned the previous verse.
Therefore the use of this verse to support the Filioque does not stand.
On the other hand, even if the verse was ambiguous, Christ states rather bluntantly in St. John 15:26"But when the Comforter is come, whom I will send unto you from the Father, [even] the Spirit of truth, which proceedeth from the Father, he shall bear witness of me"
Here we have the Orthodox Catholic missionary sending, that is, Christ sends the Spirit in a manner analogous to how He sent the Apostles and the Orthodox Catholic sentence that was used as reference for the Symbol of Faith. If there was any such identiy between Father and Son, this would be the place for Christ to have said it. Not only He didn't, but He made a clear distinctiong between the Son sending missionarily the Spirit to the world and procession from the Father.
Plus, the ex post fact RC explanation for the filioque (that it is a sending analagous but different from that of the Father) does not hold simply because both Father and Son are subjects of the same verb of which the Spirit is the object. In no context one would understand in the sentence "They are from U.S. and Canada" that this "are" means for US that they originated their and for Canada that they were just passing through the country.
Finally, the Symbol is written using the figure of speech of parallelism. It describes the Father, then the Son, then the Holy Spirit describe for each precisely that which differentiates each from the other:
- Father Almighty, Maker of heaven and earth, and of all things visible and invisible.
- one Lord Jesus Christ, the only-begotten Son of God,
- the Holy Ghost, the Lord and Giver of life, who proceedeth from the Father,
So, be begotten of God is exclusive of the Son
To proceed from the Father is exclusive of the Holy Spirit.
What is then exclusive of the Father? He is All-Mighty, the only one who is the cause of all potential. *Only* Him, can be cause to other persons of the Trinity. What is that then, that comes *through* the Son?
- by whom all things were made;
That is, what comes *through* the Son is only the created world. Not persons of the Trinity.
Finally, once these three distinctions were established: a Father Who is the only "source" of both created and uncreated things, a Son Who is begotten from the Father and a Spirit who proceeds from the Father (and here quoting Christ Himself), their essential unity is expressed in:
- (the Spirit) with the Father and the Son together is worshiped and glorified
The inclusion of "and of the Son" breaks the intended paralellism because it puts the Son and the Father doing the same action and therefore destroying any difference between them.
Now, and this is not meant as an offence, it is not surprising that the this inclusion would be forced upon the Western Church by an illiterate emperor. While in Toledo where this inclusion was first introduced and tolerated by ekonomia of the Primate, they were probably well aware of the difference and the context in which it was being done, the imposition of this element in places where there was no heresy about the Holy Spirit was itself a heresy and a corruption of the very words of Christ.
In very prosaic terms, IMO what I think that happened was that the East was used to heretical emperors trying to tamper with the Faith and the Church learned to recover with time. In the West this had long been forgotten, so when a new emperor emerged imposing this heresy, there was no experience nor humbleness posteriorly to admit that a secular power had messed with the faith and that it could and should be corrected.