St Athanasius Defends the Creed
"But if God be simple, as He is, it follows that in saying "God" and naming "Father," we name nothing as if about Him, but signify His substance itself. For though to comprehend what the substance of God is be impossible, yet if we only understand that God is, and if Scripture indicates Him by means of these titles, we, with the intention of indicating Him and none else, call Him God and Father and Lord… Therefore let no one be startled on hearing that the Son of God is from the substance of the Father; rather let him accept the explanation of the Fathers, who in more explicit but equivalent language have for from God written "of the substance." For they considered it the same thing to say that the Word was of God and "of the substance of God," since the word "God," as I have already said, signifies nothing but the substance of Him Who Is. If then the Word is not in such sense from God, as to be Son, genuine and natural, from the Father, but only as creatures because they are framed, and as all things are from God, then neither is He from the substance of the Father, nor is the Son again Son according to substance, but in consequence of virtue, as we who are called sons by grace. But if He only is from God, as a genuine Son, as He is, then let the Son, as is reasonable, be called from the substance of God".
– from St Athanasius' defense of the Nicene Creed.
Today is the feast of St Athanasius who died today in 373. This stained glass window by Connick is in Grace Cathedral, San Francisco.