I don't see {Frederick} Douglass's call for everyone to be converted to Christianity in either of DOC's links...
Well I said I was done with this issue of slavery if everyone else was but I guess that is not the case.
It's strongly implied in his autobiography when he says this:
I consulted a good colored man named Charles Lawson, and in tones of holy affection he told me to pray, and to "cast all my care upon God." This I sought to do; and though for weeks I was a poor, broken-hearted mourner, traveling through doubts and fears, I finally found my burden lightened, and my heart relieved. I loved all mankind, slaveholders not excepted, though I abhorred slavery more than ever.
I saw the world in a new light, and my great concern was to have everybody converted. My desire to learn increased, and especially, did I want a thorough acquaintance with the contents of the Bible. I have gathered scattered pages of the Bible from the filthy street-gutters, and washed and dried them, that in moments of leisure I might get a word or two of wisdom from them. While thus religiously seeking knowledge, I became acquainted with a good old colored man named Lawson. This man not only prayed three times a day, but he prayed as he walked through the streets, at his work, on his dray--everywhere. His life was a life of prayer, and his words when he spoke to any one, were about a better world. Uncle Lawson lived near Master Hugh's house, and becoming deeply attached to him, I went often with him to prayer-meeting, and spent much of my leisure time with him on Sunday. The old man could read a little, and I was a great help to him in making out the hard words, for I was a better reader than he. I could teach him "the letter," but he could teach me "the spirit," and refreshing times we had together, in singing and praying. These meetings went on for a long time without the knowledge of Master Hugh or my mistress. Both knew, however, that I had become religious, and seemed to respect my conscientious piety.
http://www.classicapologetics.com/special/slaverevolt.html
And all of this can be considered some evidence for the truth of the NT because it shows how the power of the gospel can transform the life of a poor brokenhearted slave into becoming a great man who met Lincoln and became the most well known black man in America. Also Martin Luther King's grandparents were slaves and his father and grandfather were ministers like he was.
Could phony gospels written by liars 2000 years ago have had this kind of power to help produce these great men out of broken hearted poor slaves?