What "theology" do you presume Luther invented? I'm an Evangelical Protestant - though I reject labels like those - and I can prove EVERY doctrine I hold in my Christian faith from Sacred Scripture. Can Roman Catholicism do that?
The Early Church Fathers you like to claim didn't always agree with each other in their commentaries and teachings, but they DID use Holy Scripture as their rule of faith and many disputed heresy by the more knowledgable application of Divinely-inspired Scripture.
How do you think those within the Christian faith communities determined what were the major tenets of Christianity? We have today the "more sure word of prophecy" because we have the Holy Spirit inspired and preserved revelation of those truths. Our assurance that they are true is based upon the Holy Spirit speaking through them to the hearts of those who listen for their shepherd and follow him. His truth is established in word and in power.
Jesus said, "My sheep listen to My voice; I know them, and they follow Me. I give them eternal life, and they will never perish. No one can snatch them out of My hand." (John 10:27,28)
Protestant "proofs" are either points no one disagrees with them, or peculiar interpretation of verses taken out of context. "Faith alone" is one such theological fantasy.
How do you think those within the Christian faith communities determined what were the major tenets of Christianity?
Why, the Catholic priests taught them according to the Sacred Tradition of the Church. Yes, they taught with the voice of Christ (John 10:27-28).