Genestealer Cults forces do not operate like normal armies, as they represent the fanatical members of the underground revolution masking itself as a religion. They combine strong hypnotic and genetic ties between the troops and their leaders with a near total lack of military training or properly maintained weaponry. This results in several special rules for Cults in battle.
Genestealer Cult Special Rules
Source: Codex: Genestealer Cult, p. 4