Complex sentence: Dogma is the established belief or doctrine held by a religion, ideology or any kind of organization: it is authoritative and not to be disputed, doubted, or diverged from, by the practioner or believers.
Split into simpler sentences: Dogma is the established belief or doctrine held by a religion, or by extension by some other group or organization. It is authoritative and not to be disputed, doubted, or diverged from, by the practioner or believers.