Sentence:  The brazen serpent, lifted up in the centre of Israel's camp, with the publick declaration of its use, was a sufficient <WSD>"warrant</WSD>" to every man, when bitten by a fiery serpent, to look unto it. But [...] if any looked without at all expecting a cure according to the word of the Lord, they must have perished; not for want of a <WSD>"warrant</WSD>" to believe; but because they did not submit to the wisdom and authority of God, or rely on his faithfulness and mercy, in this appointed way of preservation. 
Meaning of the word marked with <WSD> tags:  Something that provides assurance or confirmation; a guarantee or proof.