Patrick Henry's "Speech to the Virginia Convention" is an important speech for colonial America.  The speech is Henry's thoughts on what to do about the "British Problem." The British problem, if you need a little background, is the no taxation without representation stuff from history classes. The colonists either wanted parliamentary representation or they wanted to be able to separate from Britain and begin making their own rules and laws.  Third option is all out rebellion and war, which is what happened.  

A slavery allusion comes when he says "I consider it nothing less than a question of freedom or slavery." "It" is referring to the topic of American independence. For Henry there is no middle ground.  It's either fight and be independent (free) or be ruled by Britain (slavery).  Listeners at the time would have been strong in their Bible knowledge and they would see the connection to Israel's slavery in Egypt and their forceful removal from it. Listeners would also have noticed that Henry doesn't believe in a middle ground, which could reference Revelations 3:16 in which the lukewarm people are spit out. The colonists wouldn't want to displease the eyes of God, so they must choose freedom or slavery. And when Henry presents those as the only two options, the choice is pretty clear what he wants most people to side with.