The English word “theology” is etymologically derived from the Greek noun θεός, meaning “god,” and the suffix -λογια, meaning “study of.” Thus, theology is the study of god. On this site, it refers to either the study of god as a nature,1 or the study of god as the person2 of “God the Father.” St. John of Damascus wrote,3
“We have frequently stated that essence (οὐσία) is one thing and hypostasis (ὑπόστασις) another, and that essence (οὐσία) signifies the common and general species (εἶδος) of hypostases of the same species (ὁμοειδῶν), such as “God” (θεός), “human” (ἄνθρωπος), but hypostasis (ὑπόστασις) signifies the individual, that is to say, “Father,” “Son,” “Holy Spirit,” “Peter,” “Paul.” Well then, one must know that “divinity” (θεότητος) and “humanity” (ἀνθρωπότητος) are names of essences (οὐσιῶν) or natures (φύσεών), but “God” (θεὸς) and “human” (ἄνθρωπος) are applied in reference to natures (φύσεως), whenever we say, “God is an incomprehensible essence (οὐσία),” and “God is one.” But, it is also understood of in reference to hypostases (ὑποστάσεων) when the name of the more general is applied to that which is more specific, as when the scripture says (Psa. 45:7), “Therefore, O’ God, your God has anointed you...” (for behold, it indicates the Father and the Son), and as when it states, (Job 1:1), “There was a certain man in the land of Uz” (for, it only indicated Job).”
Ὅτι μὲν οὖν ἕτερόν ἐστιν οὐσία καὶ ἕτερον ὑπόστασις, πλειστάκις εἰρήκαμεν, καὶ ὅτι ἡ μὲν οὐσία τὸ κοινὸν καὶ περιεκτικὸν εἶδος τῶν ὁμοειδῶν ὑποστάσεων σημαίνει οἷον θεός, ἄνθρωπος, ἡ δὲ ὑπόστασις ἄτομον δηλοῖ ἤτοι πατέρα, υἱόν, πνεῦμα ἅγιον, Πέτρον, Παῦλον. Ἰστέον τοίνυν, ὅτι τὸ μὲν τῆς θεότητος καὶ τῆς ἀνθρωπότητος ὄνομα τῶν οὐσιῶν ἤτοι φύσεών ἐστι παραστατικόν, τὸ δὲ θεὸς καὶ ἄνθρωπος καὶ ἐπὶ τῆς φύσεως τάττεται, ὁπόταν λέγωμεν· Θεός ἐστιν ἀκατάληπτος οὐσία, καὶ ὅτι εἷς ἐστι θεός· λαμβάνεται δὲ καὶ ἐπὶ τῶν ὑποστάσεων ὡς τοῦ μερικωτέρου δεχομένου τὸ τοῦ καθολικωτέρου ὄνομα, ὡς ὅταν φησὶν ἡ γραφή· «Διὰ τοῦτο ἔχρισέ σε ὁ θεὸς ὁ θεός σου» (ἰδοὺ γὰρ τὸν πατέρα καὶ τὸν υἱὸν ἐδήλωσε), καὶ ὡς ὅταν λέγῃ· «Ἄνθρωπός τις ἦν ἐν χώρᾳ τῇ Αὐσίτιδι» (τὸν γὰρ Ἰὼβ μόνον ἐδήλωσεν).
John of Damascus (Ἰωάννης ὁ Δαμασκηνός). “An Accurate Exposition of the Orthodox Faith” (Ἔκδοσις Ἀκριβὴς τῆς Ὀρθοδόξου Πίστεως). Patrologiæ Cursus Completus: Series Græca Prior. Ed. Migne, Jacques Paul. Vol. 94. Petit-Montrouge: Imprimerie Catholique, 1857.
1 Greek φύσις, οὐσία; Latin natura, essentia
2 Greek πρόσωπον, ὑπόστασις; Latin persona, substantia
3 Book 3, Ch. 4 (997-998)