Usually, when Christ told a parable, or an allegory, or made some other representational statement, and the disciples didn't understand, he explained - he said, "this is what I meant - the seed is God's word, etc. etc."
But in John 6, he says "verily I say unto you, except ye eat the flesh of the Son of Man, and drink his blood, there is no life in you. Whoso eateth my flesh and drinketh my blood, hath eternal life, and I shall raise him up on the last day. For my flesh is meat indeed, and my blood is drink indeed. Whosoever eateth my flesh and drinketh my blood, dwelleth in me and I in him." The disciples murmur among themselves, taking it quite literally, Christ does NOT say, "you misunderstand - here is the explanation." He repeats his statement, even more emphatically. And even when many of his disciples turned away and "walked with him no more," he did not correct the statement.
The Greek word translated as "indeed" is alhqhV - "truly, actually, really, I'm telling you the truth."
It's not just St. Athanasius . . . it is ALL the Early Church Fathers, without exception. The idea that the Eucharist is somehow merely a symbolic act or a fellowship meal is quite a modern idea. Even C.S. Lewis believed in the Real Presence, and felt it was necessary for modern Christians as part of the inclusion of both mystery religions and ethical religions in Christianity.
You have a link? I've read a bit of C.S. Lewis and would be a bit surprised if he made such a direct statement.