A militant is someone who is engaged in a war or who acts aggressively for their cause. If you are militant in your beliefs, you do not question them anymore than a soldier questions his orders.
In the context of undeclared or guerrilla warfare, enemy fighters are often referred to not as soldiers but as militants, because they are not members of a formal army. As an adjective, militant often has a negative feel–-the feminist movement was set back by the far right's depiction of the militant feminist.
