[Cronos]

I was later convinced that the Orthodox understanding of the Holy Trinity differs substantially.

I disagree. The Orthodox do NOT differ from us on the basic understanding of the Holy Trinity -- the question of the filioque is not that the Orthodox church considers Christ to be a 'junior partner' in the Trinity, nor that the Catholic Church considers Christ and the Father to BOTH be the sources for the Holy Spirit. No, both churchs understand the same idea: that of a triune God, but there is substantial misunderstanding in what the Western Church means by 'We believe in the Holy Spirit, the Lord, the giver of Life, who proceeds from the Father, AND THE SON' (of course, the portion in caps is not accepted by the OC as they think it means a dual origin of the Holy Spirit, while the Catholic Church will hasten to say it does not.

Do note that ALL the rites of the Catholic church do not add the filioque -- AFAIK, it is only the Latin rite that does so

[annalex]

The Orthodox do NOT differ from us on the basic understanding of the Holy Trinity

[...]

the Catholic Church will hasten to say [Filioque] does not [mean a dual origin of the Holy Spirit].

The Orthodox are correct about that: Catholicism teaches the dual procession of the Holy Spirit, and the Orthodox reject it. See

Filioque

, where you'll find the relevant catechism and a very straighforward chapter from the Summa.