Is Islam a cult

The claim is designed to reclassify rather than to argue. It reduces adherents to cultists and thereby avoids engaging with anything they believe.

The sociological definition

In the sociology of religion a cult is defined as a religiously specialised subgroup within a society, a sect or a church. The definition is descriptive, and applying it to a global tradition with more than a billion adherents, its own legal schools, and fourteen centuries of scholarship empties the word of any content it was supposed to carry.

The lexical definition

Religion is defined as belief in the existence of a supernatural ruling power, the creator and controller of the universe, who has given to man a spiritual nature which continues to exist after the death of the body. Islam’s doctrine of tawhid (the oneness of God) and its eschatology satisfy that definition directly and without strain.

What the label is for

The term is chosen for its connotation rather than its meaning. It carries an implication of manipulation and smallness, and it is deployed to secure that implication without having to defend it.

A claim that fails against both the sociological definition and the dictionary is doing no descriptive work at all. It is doing rhetorical work, and it is worth asking why the argument requires it.