c. 390
Greek
On Not Three Gods
Ad Ablabium
Gregory of Nyssa · c. 335 – c. 395
A short answer to the obvious objection: if Peter, James, and John are three men, why are Father, Son, and Spirit not three Gods. Gregory replies that calling three men three is already a loose habit of speech, and that the divine persons are one because every divine action begins in the Father, proceeds through the Son, and is completed in the Spirit as a single motion.