Beginning with the nailing of his 95 Theses on the Wittenburg Church door by Martin Luther on October 31, 1517 (so the 500th anniversary is approaching), Protestants have systematized our conflict with Rome in the so-called Five Solas:
Sola Scriptura: that our only infallible standard for spiritual truth is the Holy Bible, in the Old and New Testaments, and what by necessary logical consequence might be based on them.
Soli Deo Gloria: that our salvation and sanctification are for the glory of God alone, not based on any works for which a man might claim credit.
Solus Christus: that our salvation is based on the finished work of Christ alone, in His life, crucifixion, resurrection, and eternal intercession.
Sola Gratia: by grace alone, that is, by God's voluntary condescension, not because of any obligation that we have placed upon Him.
Sola Fide: that it is by faith alone, as the instrumental means, that the works of Christ are imputed to us for our justification.
These Five Solas (Latin for "alone") are the essential points of conflict between the churches of the Reformation and the Church of Rome. She has never changed her denial of these five truths, so our repudiation of her legitimacy must be maintained, for the rest of human history, if need be. Any ecumenical relationship, while she continues in her apostasy from the Gospel, can only carry Protestants into judgment with her. As Jesus Himself says (Revelation 18:4): "Come out of her, My people, lest you take part in her sins, lest you share in her plagues."