Participation is a theme which you'll find throughout patristic theology (that's theology written by old dudes). It refers to the idea that everything that exists gets its particular properties (goodness, wisdom, stability, blackness, fluffiness, nakedness) from God, who is the source of everything. And everything gets those properties not to keep, but only as a sharing in God's properties – so my wisdom, wit, beauty, goodness, kindness etc. aren't absolutely mine but are my sharing in those properties in God (he's been good to me). One of the quirks of this way of understanding creation is that existence, “beingness” is counted as a property (I've heard that this raises interesting philosophical questions and difficulties, though I couldn't for the life of me tell you what), which means that to the extent that I exist, I get my existence from participation in God.
In this way of seeing the world, God's simplicity is really important. That doesn't mean that he's stupid, but that God can't be divided: we might have a whole collection of different attributes which are stuck together to make an individual, but in God those different attributes are absolutely united, so his goodness is the same as his wisdom is the same as his existence is the same as his orangeness.
This is a lot to do with Plato, and Neoplatonism (which dates from about the 3rd century AD) which has the idea of one absolutely unified principle as the source of everything, like the top point of a pyramid, and everything else radiating or emanating outwards from it like the rays of the sun, becoming more fragmented, more diverse and further from the source the further out it gets. This means that everything not only has different degrees of closeness and resemblance to the source, but also that things have different degrees of beingness. Things in the world can be more or less real, and confusingly, this is taken to mean that spiritual things and qualities – intellect, goodness, beauty – are more real than more physical and tangible things like rocks and pencils.
Lots of interesting consequences ensue. It means that everything in creation, however big or small, significant or insignificant, bears some sort of resemblance to God. It means that we can be more or less real, more or less existent, and that the more we come to resemble God, the more we participate in him and his goodness. It means that every attribute of everything that there is bears some resemblance to God. I'll come back to this, probably repeatedly, because there's a lot of meat here, but for now I'll leave you with this thought: God is naked.