What have the fathers and doctors of the Church said or wrote about salvation outside the Church? What did St. Thomas Aquinas, St. Augustine or any other saint say about salvation outside the Church?
The Catholic Answers tract has some good quotes. If you Google “extra ecclesiam” you’ll get a bunch more, but beware that some sites will probably be (ironically) those of schismatics. You’ll also want to check out Boniface VIII’s bull Unam Sanctam, and the teaching on this subject of the Fourth Lateran Council. St. Thomas mentions it in passing in S.T. III, Q73, a3.
There is one mediator between God and man: Jesus Christ.
If there is only salvation through Jesus, then there is only salvation through the Church.
In addition to the link provided by Ad Orientem, I recommend this link:
Scripture, Church Fathers, and Medieval Doctors on the Possibility of Salvation for Non-Catholics