Today we celebrate the Solemnity of the Most Holy Trinity in the Catholic Church. This reality that we call the Triune Godhead is the main pillar which supports orthodox Christianity. It is what separates the heretic from the Catholic (universal) Christian. It is of paramount importance because tied intricately within this conception is a host of related doctrines that are benchmarks for the Christian faith. Doctrines such as the ontological definitions surrounding the Persons of the Trinity, (full deity for the Father, Son and Holy Spirit). Doctrines such as the outworking of the Trinity in redemptive history towards mankind.
The earliest Christians were full fledged Trinitarians (even the pre-Nicene writers). Even non-bishops reflected this understanding beautifully as the following quote from the philosopher Aristides of Athens1 proves:
...are called Christians...for they acknowledge God, the Creator and Maker of all things, in the only-begotten Son and in the Holy Spirit. Other than Him, no god do they worship. (Απολογια. 15)
Hence it is clear, in which a largely non-clerical writer, in a context of explication of Christian worship, clearly states the Trinitarian belief.2 This Trinitarian belief over time was codified and developed in reaction to heretical claims, crystallized in the great early Catholic Councils by the fathers of the East and West.
This is an example of development of doctrine that all branches of Christendom (Catholic/Protestant/Orthodox) hold and defend. I submit to you then if your particular church did not even mention the Trinity in today's "service," you should really question the historicity and validity of this group.
. This "Apology" for the Christians was written most likely not to Emperor Hadrian but to his successor Antoninus Pius, which had the full name of - Titus Aelius Hadrianus Antoninus Pius, who reigned from 138-161 AD. Hence this Apology is dated around 140 AD, cf. William Jurgens, The Faith of the Early Fathers (Collegeville, Minnesota: Liturgical Press, 1970, Vol. 1), pp. 48-49.
. Earlier Catholic writers are just as clear, Clement of Rome in his Letter to the Corinthians, (which is usually dated to the latter part of the first century) is clear (42:1-5; 58:2), Ignatius of Antioch, (Προς Εφεσιους επιστολη. 9:1-2, 110 AD; Προς Μαγνησιους επιστολη. 11, 110 AD) are some examples.