“But the definition of "person," as given above, does not apply to God. Both because reason implies a discursive knowledge, which does not apply to God, as we proved above (Q. 14, A. 12); and thus God cannot be said to have "a rational nature." And also because God cannot be called an individual substance, since the principle of individuation is matter; while God is immaterial: nor is He the subject of accidents, so as to be called a substance.
Therefore the word "person" ought not to be attributed to God.
On the contrary, In the Creed of Athanasius we say: "One is the person of the Father, another of the Son, another of the Holy Ghost."