Basil: Trinitarians do not believe God is “one” numerically

For everything which is called one in number is not one absolutely, nor yet simple in nature; but God is universally confessed to be simple and not composite. God therefore is not one in number. What I mean is this. We say that the world is one in number, but not one by nature nor yet simple; for we divide it into its constituent elements, fire, water, air, and earth.6 Again, man is called one in number. We frequently speak of one man, but man who is composed of body and soul is not simple. Similarly we say one angel in number, but not one by nature nor yet simple, for we conceive of the hypostasis of the angel as essence with sanctification. If therefore everything which is one in number is not one in nature, and that which is one and simple in nature is not one in number; and if we call God one in nature how can number be charged against us, when we utterly exclude it from that blessed and spiritual nature? Number relates to quantity; and quantity is conjoined with bodily nature, for number is of bodily nature. We believe our Lord to be Creator of bodies. Wherefore every number indicates those things which have received a material and circumscribed nature. Monad and Unity on the other hand signify the nature which is simple and incomprehensible. Whoever therefore confesses either the Son of God or the Holy Ghost to be number or creature introduces unawares a material and circumscribed nature. And by circumscribed I mean not only locally limited, but a nature which is comprehended in foreknowledge by Him who is about to reduce it from the non-existent into the existent and which can be comprehended by science. Every holy thing then of which the nature is circumscribed and of which the holiness is acquired is not insusceptible of evil. But the Son and the Holy Ghost are the source of sanctification by which every reasonable creature is hallowed in proportion to its virtue. (Basil, To the Caesareans, Letter VIII section 2)