Every once in a while I browse LDS.net or MADB, and find threads discussing God, and basically the nature of the Father, Son, and Holy Spirit. As most familiar with the LDS, they do not accept the Trinity, formally defined at the Council of Nicaea (including the nature of Jesus Christ). Now, I would just like to emphasize that Catholics, Orthodox, Oriental Orthodox, and others that see the Councils as authoritative, do not believe that the Trinity came into existence in 325 AD at Nicaea. Instead, we recognize that in times where heresies begin to arise, a Council can be called to formally define doctrine in the face of that heresy. So, traditional Christians believe that the Trinity has always been the Christian belief, but that in the face of subordinationism and other heresies, it was formally defined at the Council of Nicaea, and can be read in the Nicene Creed.
Now, one major misunderstanding that many LDS have with traditional Christianity is what exactly Trinity means! Many of the discussions begin with “was Jesus talking to Himself when He talked to the Father?” and “how can the Father, Son, and Holy Spirit be the same person?”
I would like to state that Catholics do not believe that the Father, Son, and Holy Spirit are the same Person. Catholics believe that the Father, Son, and Holy Spirit are three distinct, separate Persons. So when Mormons bring up “was Jesus talking to Himself” as an argument against the Trinity, it begins with a false premise, because Catholics accept that Jesus and the Father are separate Persons.
To further define this, let us look to the Catechism:
254 The divine persons are really distinct from one another. "God is one but not solitary."86 “Father”, “Son”, “Holy Spirit” are not simply names designating modalities of the divine being, for they are really distinct from one another: "He is not the Father who is the Son, nor is the Son he who is the Father, nor is the Holy Spirit he who is the Father or the Son."87 They are distinct from one another in their relations of origin: "It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds."88 The divine Unity is Triune.
So, Catholics do believe that the Father, Son, and Holy Spirit are consubstantial, or are of the same substance. However that does not mean that they are the same Person. Catholics and other traditional Christians fully believe that the Father, Son, and Holy Spirit are three separate, distinct Persons, and are also one God.