"Yikes! Your highlighted portion is certainly loaded (and heretical)."
Really? You know, Blogger, that's the usual protestant response so you're not alone! It is indeed loaded, which is why I highlighted it and likely why +Athanasius wrote it (he was a provocative sort of bishop). Why do you think that its heresy? :)
Ah So, you're picking a fight! Bad boy!
Why? Because he is saying that we become God. If he had said we become like Christ, then it wouldn't be so much of an issue. Becoming God has a different connotation to it (particularly in this New Agey 'I am god' world).
Language means something - especially surface terminology. If it takes writing a theological treatise to explain what you mean by the use of a particular phrase, it might be wise to consider whether the phrase should have been used at all.