Why did Luther react so violently to Zwingli on the one hand and the Anabaptists on the other?

He wasn’t because he insisted on his own formula for the real presence or baptism. As Jaroslav Pelikan pointed out in his 1968 Spirit versus Strcuture , “when Luther was confronted with a position that was less precise than his own about the real presence in the eucharist, but was no less firm than his about the doctrine of grace, he was willing to be charitable and patient. Thus in relation both to Martin Bucer, in the consideration of the Wittenberg Concord of 1536, and to the Unity of Bohemian Brethren, in the consideration of their Confession of 1535 which he published in 1538, Luther acts in a manner quite different from his attitude toward Zwingli.”

The reason, Pelikan says, was that he saw a commonality between the Catholic view of the Mass as a work and the sacramentarian view of the real presence or the Anabaptist view of baptism: “he . . . recognized that there was a subtle but profound affinity between this equation of sacrifice with sacrament and the rejection of all structure in the name of the Holy Spirit. Despite their obvious differences, both views seemed to him to emphasize human initiative and responsibility in the sacrament at the cost of divine grace.”

For Luther, one could not oppose distorted sacraments with no sacraments, mistaken real presence with a denial of real presence. The only alternative to both Catholic and Anabaptist was a reformed understanding of real presence, combined with a reformed mass.