A certain Uniate priest by the name of John came to doubt the truth of his confession and appealed to Elder Paisius Velichkovsky with the entreaty that he clear up his perplexity. The main part of the Elder’s reply is given here:
The Holy Spirit Himself, Who proceeds from the Father and reposes in the Son, has inspired you by His grace to appeal with this question to a humble and sinful, but Orthodox, son of the Eastern Church…
The first and most important error of the Uniates is the teaching, which they have taken from the Romans, that the Holy Spirit proceeds from the Father and the Son [Filioque]. This is the first and most important of all the heresies, for it includes in itself an incorrect judgment, contrary to Sacred Scripture, about God, Who is One in the Holy Trinity. He who confesses that the Holy Spirit proceeds from the Father and from the Son supposes in God two principles: one of the Father, another of the Son. But we Orthodox confess in the Trinity one principle of the Father, as our Lord Jesus Christ taught us in the holy Gospel, when He said that the Holy Spirit proceeds from the Father alone. He said: When the Comforter is come, Whom I send unto you from the Father, the Spirit of truth, Who proceedeth from the Father (Jn. 15:26). And the Apostle says: Every good gift and evry perfect gift is from above, coming from the Father of lights (Jm. 1:17). Do you see? He says “the Father of lights”; that is, the Father is the root and fount of Divinity; and the two lights, the Son and the Spirit from the single light, the Father, have their pre-eternal being, the Son in being begotten and the Holy Spirit in procession.
The divine Prophet David says: By the word of the Lord were the heavens established, and all the might of them by the Spirit of His mouth (Ps. 33:6). Do you see? He calls the Father Lord, but he calls the Son Word, as pre-eternally begotten of Him; and he calls the Holy Spirit of His (and not “Their”) lips, as proceeding from the Father alone. One could search out other testimonies also from the Old and New Testaments, which show more clearly than the sun that the Holy Spirit proceeds from the Father alone and reposes in the Son, as was disclosed at the Baptism of the Lord.
Further, all the holy ecumenical teachers who have interpreted the Scripture as if with one mouth say that the Holy Spirit proceeds from the Father, and nowhere have they written that he proceeds from the Son also. Thus, if Uniates think exactly like the Romans in such a serious heresy, what hope do they have for salvation, unless they openly renounce this Spirit-fighting heresy and become united again with the Holy Orthodox Eastern Church?
Spare neither property nor relatives if they do not wish to listen to you, but by all means save your own soul from perdition; because there is nothing more needful for you than the soul for which Christ died. But in fleeing, do not look back in your heart for the sake of swiftly perishing possessions; it is better for you to remain in poverty than to blaspheme Him. Depart and flee from the Unia as speedily as possible, lest death overtake you in it and you be numbered among the heretics and not among the Christians. And not only go away yourself, but advise others to go away also, if in your conscience you know that they will hear you. And if they will not hear you, then at least depart yourself from the nets of the enemy and be united in soul and heart with the Holy Orthodox Church, and thus, together with all [the faithful] holding the inviolate faith and fulfilling the commandments of Christ, you will be able to be saved. (The Letters of Elder Paisius from Niamets II: To a Uniate Priest, on the Procession of the Holy Spirit. Blessed Paisius Velichkovsky pp. 200-202)