14 Now first, I ask, what is the meaning of these terms, 'true God?' and 'not true God?'? If any one says to me 'This is fire, but not true fire; water, but not true water,' I can attach no intelligible meaning to his words. What difference in kind can there be between one true specimen, and another true specimen, of the same class? If a thing be fire, it must be true fire; while its nature remains the same it cannot lose this character of true fire. Deprive water of its watery nature, and by so doing you destroy it as true water; let it remain water, and it will inevitably still be true water. The only way in which an object can lose its nature is by losing its existence; if it continue to exist it must be truly itself. If the Son of God is God, then He is true God; if He is not true God, then in no possible sense is He God at all. If He has not the nature, then He has no right to the name; if, on the contrary, the name which indicates the nature is His by inherent right, then it cannot be that He is destitute of that nature in its truest sense.
Source: On the Trinity (New Advent)