Which religious issue divided Martin Luther and Huldrych Zwingli at the Colloquy of Marburg?

At the Colloquy of Marburg in 1529, the primary religious issue that divided Martin Luther and Huldrych Zwingli was the nature of Christ’s presence in the Eucharist.12

The disagreement centered on the interpretation of Christ’s words, "This is my body". Luther defended a literal understanding, maintaining the real presence of Christ’s body and blood "in, with, and under" the bread and wine. Conversely, Zwingli argued that the Eucharist was a symbolic memorial rite, contending that the word "is" meant "signifies" and that Christ was present only spiritually to the believer.13

Philip, landgrave of Hesse, organized the colloquy in hopes of resolving this theological divide to form a unified Protestant political alliance against the Holy Roman Empire. Despite reaching agreement on most other doctrines, the reformers could not reconcile their views on the sacramental presence, and the theological impasse ultimately prevented the formation of the comprehensive alliance Philip had envisioned.123