Why did Philip and other Protestant leaders protest at the Diet of Speyer in 1529?

At the second imperial Diet of Speyer in 1529, Philip, landgrave of Hesse, and other Protestant leaders protested against the repeal of the 1526 imperial decree.12

The 1526 decree had previously granted each imperial estate the freedom to govern religious matters in a way they could justify before God and the emperor, which had allowed Lutheran rulers to proceed with their church reforms. However, following military successes, the emperor’s representative, Ferdinand, demanded that this earlier concession be rescinded and that religious dissenters end their separation pending a general council.13

In response, a group of princes and cities issued a formal protest against this abrogation of the 1526 decree. Their opposition was grounded in the principle that, in matters of faith, each imperial estate must justify itself before God alone. This historic act of defiance is the origin of the term "Protestant," which subsequently became the name for the followers of Luther and other Reformation theologians.123