When in Greece, I read the soberly good, soul-shattering Philokalia. Back in the West, I revert to the Summa, where I just caught Aquinas dishing out some can't-we-all-just-get-along. First, some quick background. For Aquinas, there are five "notions" in God, which signify not God's essence, but are instead ideas that help us fathom the individual persons of the Trinity. Among these notions is the idea of procession, under which we can file the filoque disagreement that has long contributed to the East/West divide. While clearly defending the western side of the issue, Aquinas nevertheless brings himself to write the following:
Notions are not articles of faith. Therefore different opinions of the notions are permissible.... we must decide that anyone may entertain contrary opinions about the notions if he does not mean to uphold anything at variance with faith. If, however, anyone should entertain a false opinion of the notions, thinking that consequences against the faith would follow, he would slip into heresy.In other words, whatever your opinion on the church-dividing filioque clause, if by holding it you aim not to intentionally undermine Christian faith, you've hit the ball within the foul lines. Call that the heresy of ecumenism, or the better part of wisdom.