I think that Frederick Douglass thought that it was very important to speak publicly about the issues of slavery. By speaking about it and getting the truths of slavery out in the open would be a way to open peoples eyes. Douglass published a paper called the "North Star" which was an anti-slavery newspaper. He kept printing this paper all the up until the Emancipation Proclamation.
Frederick Douglass also believed that the Constitution itself could be used as a device to abolish slavery. This greatly angered his friend and fellow abolitionist William Lloyd Garrison who actually burned copies of the document in protest.
In his speech on the Fourth of July, 1852, Frederick Douglass discusses a couple of ways in which, he says, slavery could be or will be abolished.
First, he says that the church could abolish slavery if it wanted. He talks about how the church helps to prop up slavery. And then he says that if the church would work hard to oppose slavery, slavery could never continue. As he says
Let the religious press, the pulpit, the Sunday school, the conference meeting, the great ecclesiastical, missionary, Bible and tract associations of the land array their immense powers against slavery and slave-holding; and the whole system of crime and blood would be scattered to the winds
A second argument that he makes is that slavery is inevitably going to end. This argument is made at the end of the speech. He says that civilization is just growing too much and that public opinion around the world is so opposed to slavery that slavery cannot keep existing.