Patrick Henry's speech is liberally sprinkled with biblical quotations and allusions, and this rhetorical question is one of them. As a rhetorical question, this interrogatory implies a negative answer. Henry suggests those assembled do not fit this description.
The words Henry quotes were spoken by Jesus to his disciples in Matthew 13:13. The disciples asked Jesus why he spoke to the people in parables and not directly, and Jesus responded:
This is why I speak to them in parables: "Though seeing, they do not see; though hearing, they do not hear or understand."
He goes on to say that the people have hard hearts and don't want to understand. They have willfully closed their eyes and ears to the message of God, so they cannot be healed--or saved. But Jesus commends his disciples for being willing to see and hear the gospel.
An allusion is powerful because it carries the context of a larger story or situation in just a few words. Henry and his listeners would have been familiar with Jesus' words and their context. The allusion suggests that the members of the convention are like the disciples; because they are willing to see and hear the reality of the threat before them, they will be blessed and obtain salvation--in this case, freedom from Britain, a temporal rather than spiritual salvation.
Looking at the rhetorical device of the quoted words, we see that Jesus used, and Henry repeated, a paradox. A paradox is something that seems contradictory or unexpected. The purpose of eyes and ears is to see and hear; it would be silly for someone to refuse to use such critical senses. No one would willingly choose to be blind or deaf, but Henry says that if they refuse to take the actions of Britain seriously, they will be that foolish.
In another sense, we can consider this rhetorical question to be hyperbole. It is figurative language rather than literal. He's not suggesting the delegates will be literally blind or deaf, but he is exaggerating to make his point.
Thus in this rhetorical question, Henry uses allusion, paradox, and hyperbole to stir the hearts of his fellow delegates to the Virginia Convention.