There is no enumeration of things which has the divine nature as a member. In this sense, God is not one (this is the one of quantity or number). Again, we cannot include God in some enumeration of things such that the whole enumeration would be greater than the single member, God. This is true even when speaking of the persons of the Trinity- and is true a fortiori of God and creatures.
The absolute oneness of God follows from the denial of any of the division we find in sensible and imaginable creatures. It follows that reason must allow for a the trinity of persons as a real possibility, since reason proves the unity of God by negating all division found in creatures as found in creatures, but not all division as it may be found in something other than a creature.