The meaning of the example is this: The devil is the strong man; his goods are the men into whom he is received; unless therefore a man first conquers the devil, how can he deprive him of his goods, that is, of the men whom he has possessed?
So also I who spoil his goods, that is, free men from suffering by his possession, first spoil the devils and vanquish them, and am their enemy. How then can ye say that I have Beelzebub and that being the friend of the devils, I cast them out?
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).