I agree wholeheartedly with what you say about the dangers of ecumenism. Though I’m on record as an “esoteric ecumenist”, I always try to make it quite clear in my writings that I remain opposed to most forms of contemporary interfaith dialogue, not only because of the “reductionism” you mention but also because of the participants’ soporific sentimentalism.
You don’t deny, you say, that God’s grace is bestowed universally, but you wish to contend that it acts “inwardly” only among Christians and “merely outwardly” when it comes to the adherents of other religions. I would be interested to know what criteria you propose to apply in testing this hypothesis. How would you go about assessing the difference between, say, the Muslim shaykh Ahmad al-Alawi (see Martin Lings’s Sufi Saint of the Twentieth Century) and the Christian geronda Joseph the Hesychast? Your position reminds me of a chapter called “Is There a Natural Mysticism?” in Schuon’s book Gnosis: Divine Wisdom, where he calls attention to the “optical illusion” inescapably involved when we endeavor to measure the “degrees” of grace in another religion from the vantage point of our own. To use a memorable C. S. Lewis image: the telegraph pole we’re standing next to always appears larger than the other poles on down the line.
As for your claim that “perennialism as such is not found in the Fathers”, I completely agree. I would go even further and add that there are very few, if any, “classical” Christian writers of any period—Patristic, Medieval, or Modern—who provide explicit support for believing that “the Holy Spirit is salvifically operative in the sacraments of non-Christian religions” (to quote you again). Does this mean the “giants” of our tradition were themselves subject to this illusion of perspective? Perhaps. But if so, given the amount of information most of them had concerning other religions, they’re certainly not to be criticized for this limitation. And besides, as has been pointed out more than once by perennialist writers, religions are in the business of saving as many people as possible. Since the “many” tend to be deaf to metaphysical subtleties, I don’t see how one could, or should, expect anything else of a religion’s leading theological spokesmen.
In his Christianity: Lineaments of a Sacred Tradition, Philip Sherrard—a fellow Orthodox perennialist, as you know—points out that “from the beginning there appear to be two different attitudes to the non-Christian world”: an “entirely negative attitude”, inherited from Judaism, and a “more positive attitude”, represented (as Sherrard sees it) by Justin Martyr, Clement of Alexandria, Maximus the Confessor, Augustine, and others (see pp. 56-57). But even then, if one studies the passages from these latter authorities cited by Sherrard, it’s clear they were speaking only of pre-Christian traditions, and only of the breadth of God’s mercy in general.
I can therefore well appreciate your feelings of conflict on this matter, your sense of the danger involved in going against, or at least seeming to go against, the teachings of Orthodox Tradition. How can the perennialist be a “true believer”, you ask? I answer that question for myself in this way: I can be a true believer only by believing the Truth, wherever it might be manifest, whether in my religion or in another. There are simply too many Shaykh al-Alawis (Muslim and otherwise) for me to think that Christianity alone bears the fruit of great saints. My advice to you, however, given the obvious depth of your concerns, is to set the perennialist perspective aside and immerse yourself in the treasures of Orthodoxy. Genuine perennialists—as distinct from the pluralists who populate the interfaith symposia—would be the first to tell you that you’d be losing nothing, but standing to gain Everything.