Why did the Colloquy of Marburg fail to create a united Protestant alliance?

The Colloquy of Marburg failed to create a unified Protestant alliance primarily because of an irreconcilable theological disagreement regarding the Eucharist.12

While Philip of Hesse sought a political confederation to protect Protestants against potential Roman Catholic aggression, he recognized that Lutheran leaders required a common confession of faith as a prerequisite for such an alliance. During the 1529 meeting, Martin Luther insisted on a literal interpretation of Christ's words, "This is my body," maintaining that Christ was physically present in the bread and wine. Conversely, Huldrych Zwingli argued that the Eucharist was a symbolic memorial and that the word "is" signified "signifies".123

Although the participants reached agreement on most other theological issues, this critical gulf remained. Luther ultimately refused the hand of fellowship extended by Zwingli and Martin Bucer, preventing the formation of the comprehensive defensive alliance Philip had envisioned.32