"What about 2 Thess 2:15?"

That verse actually proves my point further. Paul exhorts the Thessalonians to stick with what they have already received only. It is preaching against new traditions and demanding them to stick only to what was already given. No doctrinal development.

"Orthodox gave you the Bible?"

The Bible texts are not authoritative because they are recognized by the Church, but rather the Church recognizes them because the texts themselves are authoritative. Corrupt Pharisees passed down the OT canon but their traditions were not infallible, similar to the Orthodox.

"Why do you believe the Bible?"

“I choose to believe the Bible because it’s a reliable collection of historical documents written by eye witnesses during the lifetime of other eye wittinesses. They report supernatural events that took place in fulfillment of specific prophesies and claim that their writings are divine rather than of human origin.”

"Why not Apocrypha?"

1 Maccabees 9:27 denies the idea that the Apocrypha is divinely inspired

"Did early Christians agree with this?"

Yes. Ignatius said,

“I do not, as Peter and Paul, issue commandments unto you. They were apostles;”

(The Epistle of Ignatius to the Romans, Ch. 4)

"Our Magisterium is infallible, we can't be wrong"

Not biblically accurate, in reality. OT consistently makes promises to Israel that get broken because of the nations unbelief or disobedience. To think your church can't possibly go wrong is disrespectful to God. No men are infallible. If we say we have no sin we deceive ourselves and the truth is not in us. All fall short of the glory of God. Including Mary.