“If this religion is true, why is the Muslim world in the state it is in? The decline feels like evidence.”

The observation is accurate and the inference is not, and it helps to hold those apart.

The decline is real. Muslim thinkers have said so more bluntly than any polemicist: the intellectual and methodological decline of the ummah is the core of the malaise, the educational systems are caricatures of imported models, and centuries of stagnation have spread illiteracy and superstition where scholarship used to be. Nobody serious disputes the diagnosis.

The question is what the diagnosis is evidence of. A community's condition indicts its creed only if the creed produced the condition, and the history runs in the opposite direction: the civilisation declined as it abandoned its own intellectual tradition, its educational depth, and the balance between the knowledge every person owes (fard al-'ayn) and the knowledge a community holds collectively (fard al-kifayah). The height of Islamic civilisation and the depth of Islamic learning were the same centuries. That is a strange fact for the religion to explain if the religion is the disease.

Test the inference on anything else. Every tradition on earth has presided over a nadir, and if the worst centuries falsify the creed, nothing anyone has ever believed survives. An argument that eliminates every position including its own is not evidence. It is a mood.

The full rebuttal below walks through the diagnosis properly, with the sources. The condition of the ummah is a reason for grief and for work. It is a poor reason for a verdict.