Was there a good point (at least not a heretical point) made in the (sometimes dreaded) sermon for your Trinity Sunday Mass of Obligation?
The dogma of the Most Holy Trinity is not only a mystery, it is a really difficult mystery (…is there any other kind?).
In their zeal to help people understand, some priests go to the zoo when talking about the Trinity. You will sometimes hear priests slip into the heresy of Modalism by suggesting that the Trinity is like water, which can be found in the forms or modes of steam, ice or water. Others blithely channel their heresy of Partialism and assert that the Trinity is like an egg, which is composed of shell, yolk and white, three distinct parts that make a whole. Yet others lapse into Tritheism when they compare the Trinity to three wine bottles which, though separate, contain the same wine. Then there are the creative, but certainly heretical, proponents of Arianism who proclaim that, in the Trinity, the Father is like your planet’s yellow Sun, the Son like light the Sun produces, and the Spirit like its warmth. There are other heresies out there too, but these are common.
A solid review of the Athanasian Creed can help you sniff out heresies (and, if you are a priest, avoid preaching them). Here is the part of the Athanasian Creed which concerns the Trinity:
The Catholic faith is this: That we worship one God in Trinity, and Trinity in Unity; Neither confounding the Persons; nor dividing the Essence.
For there is one Person of the Father; another of the Son; and another of the Holy Ghost. But the Godhead of the Father, of the Son, and of the Holy Ghost, is all one; the Glory equal, the Majesty coeternal. Such as the Father is; such is the Son; and such is the Holy Ghost.
The Father uncreated; the Son uncreated; and the Holy Ghost uncreated. The Father unlimited; the Son unlimited; and the Holy Ghost unlimited. The Father eternal; the Son eternal; and the Holy Ghost eternal. And yet they are not three eternals; but one eternal. As also there are not three uncreated; nor three infinites, but one uncreated; and one infinite.
So likewise the Father is Almighty; the Son Almighty; and the Holy Ghost Almighty. And yet they are not three Almighties; but one Almighty. So the Father is God; the Son is God; and the Holy Ghost is God. And yet they are not three Gods; but one God.
So likewise the Father is Lord; the Son Lord; and the Holy Ghost Lord. And yet not three Lords; but one Lord. For like as we are compelled by the Christian verity; to acknowledge every Person by himself to be God and Lord; So are we forbidden by the Catholic religion; to say, There are three Gods, or three Lords.
The Father is made of none; neither created, nor begotten. The Son is of the Father alone; not made, nor created; but begotten. The Holy Ghost is of the Father and of the Son; neither made, nor created, nor begotten; but proceeding.
So there is one Father, not three Fathers; one Son, not three Sons; one Holy Ghost, not three Holy Ghosts. And in this Trinity none is before, or after another; none is greater, or less than another. But the whole three Persons are coeternal, and coequal.
So that in all things, as aforesaid; the Unity in Trinity, and the Trinity in Unity, is to be worshipped. He therefore that will be saved, let him thus think of the Trinity.