The prophets did not believe in an abstract concept or series of concepts; instead, they believed in the Triune God. I would think that you as an Eastern Catholic would be familiar with the teachings of the Byzantine Church on this topic. The patriarchs and prophets - according to the Eastern Fathers and medieval theologians - had an experiential knowledge of the Trinity, that is, they worshipped the Trinity even if they did not formulate trinitarian credal statements. That said, I am sure that you know that it is heresy to believe that the Trinity only arose with the New Testament, or that the prophets did not know who it was that they were worshipping. Have you ever read St. Athanasios’ De Synodis? Have you ever read the Hagioretic Tome? As a fellow Eastern Catholic I would hope that you were familiar with these things, and with many other things too of course. Knowledge of God, according to St. Basil, is primarily experiential, for it is the experience of God in the liturgy that saves a man, while a discursive grasp about divine qualities - as St. Maximos explained in his treatise Quaestiones et dubia - will be possessed by the damned in hell. Knowledge as experience always has primacy over intellectual knowledge.
I’ve read the Hagioretic tome and St. Maximos’ dubia, I’ve not read
De Synodis. Your point about experiential knowledge is not something I’m going to disagree with - I
am an Eastern Catholic after all

. But the prophets did not know that the Trinity whom they had experiential knowledge of was Trinitarian. Neither does any Muslim, or believer of any other religion who has experiential knowledge of God (which, from an empirical basis alone, is difficult to deny).
I am not a Pelagian - I believe that all salvation comes through divine grace, through the Holy Catholic Church, the Body of Christ. And I know where the fullness of the Catholic Church subsists - in those churches united to Peter, the Rock upon which Christ built His Church, and also though in a wounded fashion (wounded by their schism from Rome) those churches which have preserved the fullness of the Apostolic Faith and the Holy Mysteries. And the Church subsists in an imperfect fashion among those ecclesial communities that baptize their members into the Holy Church but which perpetuate heresy and do not have apostolic succession. And the Church subsists wherever else there is holiness and salvation. I may not be a Pelagian, but I do not subscribe to the Feeneyite heresy. I know where the Church is. I do not know where it is not.
And there is very good empirical evidence for the experiential knowledge of God by non-Christians, as evidenced by the writings of men like Abu Yazid and Al Hallaj, and also texts like the Bhagavad-Gita and the Tao Te Ching.
And even if someone is not in the state of grace, one still may have natural knowledge of God. Denial of this has been condemned as heresy by the Holy See many times, at Vatican I, in the condemnations against Modernism, and in the condemnations of other theologians - Anton Gunther, Blessed Antonio Rosmini, etc. - who denied the distinction between natural and supernatural. A sinner in mortal sin may still pray to God (otherwise, how would they come to repentance and salvation again?).
And not all knowledge of God even to those in the state of grace is experiential. That God’s existence is an intellectual fact is not as important as experiential knowledge, but it’s still true.