So is it true that by affirming the filioque, he knowingly went against the creed, but is nevertheless declared to be a saint in heaven by the Holy Orthodox Church?
I suspect that St. Augustine knew exactly what was stated in the Nicene Creed.
So it is not a mortal sin to teach heresy? You can teach heresy and be a saint at the same time?
So a heretic can be a saint?
St. Augustine of Hippo taught that the Spirit came from the Father and the Son. And yet, he was declared a Father of the Church at the Fifth Ecumenical Council, held in Constantinople in A.D. 553. In the Sixth Ecumenical Council, he is called the "most excellent and blessed Augustine" and is referred to as "the most wise teacher." In the Comnenian Council of Constantinople in 1166 he is referred to as "Saint Augustine."
If St. Augustine was a heretic, then why do many in the Orthodox Church revere him as a saint and celebrate his feast day on June 15 ?
As most theologians of the church, they are mostly correct, ie to say that no one is perfect.
Why, sure, Stanley. Let's see....if, say, you're one of those Orthodox who believe that Catholics are heretics, and Jose Maria Escriva (a Catholic) is a Saint as the Catholic Church declares, then.....bingo! You've got a saint who's a heretic. Or, is it a heretic who's a saint Think I need another beer .
"Knowingly* teaching heresy would be a sin.
And I'm fairly certain that St. Augustine never claimed that the Creed said 'from the Son'
I'm not sure why this is so difficult for you, but I'll try to lay it out.
The Creed is less than a page long. St. Augustine wrote thousands of pages on Christian doctrine. In those thousands of pages, he wrote many, many things which were not in the Creed. Some of those things he was correct about, some not so much. Whatever he wrote, even the stuff he later came back and corrected himself, I am sure that at the time he wrote it, he sincerely believe it was consistent with the creed.
One of the things St. Augustine was not correct about was the relationship of the Spirit and the Son. Orthodox do not take this as 'knowingly going against the Creed' because
1) St. Augustine did not 'affirm the filioque' because the filioque had not been inserted into the Creed during his lifetime. It's likely that his writings on the topic were part of the reason that happened, but the earliest known examples of it being inserted in the Creed (or in creedal-type statements) occur well after his repose.
1b) even if St. Augustine had inserted it into the Creed, he lived before the Councils of Ephesus and Chalcedon ruled that no changes to the Creed were to be allowed. So if he had written a personal variation of the creed which included the filioque, he would have been wrong on the topic--but not wrong in placing himself above an Ecumenical Council as those Westerners who insisted on the actual filioque at later dates were.
2) There is no evidence that St. Augustine held to his opinion on the relationship of the Spirit and Son in the face of correction. If, one of his contemporaries, like, for example the Cappadocian Fathers, had gotten hold of his writings on the Spirit and sent him a correction which he rejected; or if there was any evidence that he had read their works and written against them, then 'stubborn perseverance in error' might be an issue, but there is no evidence of any thing like that occurring.
2a) St. Augustine never condemned anyone or broke communion with them over their differing with him on this topic (again, there's little evidence he even realized that his contemporaries in the East were writing much more accurate explanations).
As I already did in the other thread, I point you to the parallel of saints who lived well before the iconoclast controversy who had negative things to say about images. They had a personal opinion--that opinion was wrong, but since they never got into an argument with those whose opinion was Orthodox and supported error in the face of correction we simply acknowledge that no one but God is actually perfect.