From what I understand of Plotinus, who was one of the last big pagan Platonist philosophers of classical antiquity, he considered The One (we would say God) to be something which is like a Platonic form - the things which Platonist philosophers consider are the real, true, eternal, and unchanging - only better, even more perfect.
Importantly, a Platonist like Plotinus considers that when the intellect apprehends a form, it becomes identified with it.
Of course, like any kind of philosophy, Christian philosophers and theologians could not swallow Plotinus' philosophy Mystical union with the One whole, without criticizing portions of it that did not square with divinely revealed truth (for instance, Plotinus is pantheistic), but -- how could Christian thinkers - coming from passages like I John 3:2 in the Scriptures - look at things like the doctrine of mystical union with the One and not acknowledge that the Platonists had some valid insights into reality?
Moreover, these philosophers have been a huge influence on the language we have historically chosen to phrase our doctrine in. Here I am thinking in particular how we call the experience of heaven the Beatific Vision. You can explain what this is without recourse to Platonism, but if you are at least somewhat familiar with their ideas about intellectual vision, it makes a great deal of sense without a lengthy explanation.