We know of the book and some of its contents because religious and political authorities hired a plethora of writers and philosophers to attack and smear Marcion and those who adhered to the first and true bible. But they made the mistake of quoting from 'Antithesis' directly and because their books survived, Marcion's writings did as well.

It's interesting to note that all of the writers who attacked Marcion did so only after his death when he was unable to respond to or refute their assertions. Some, like Tertullian in his 207 A.D. books 'Adversus Marcionem, (Against Marcion)' even went so far as to attack Paul the Apostle, calling him "the apostle of heretics." Far from the 'defender of the faith' as some Marcion-haters would have you believe, Tertullian (later banished from the church) spent much time writing about women's fashions of the day and was dismissed out of hand by Saint Jerome with the words, "As to Tertullian, I have nothing else to say except that he was not a man of the church."