Justin Martyr and Paul
One should remember Justin Martyr of Syria and then Rome, who lived 100-165 AD, was a great Christian evangelist, and explained that many professed Christ but yet blasphemed God. As one scholar introduces this quote, we read:
Justin brands Marcion and the Gnostics as heretics, [but] it is not because they deny Christ: they do not. They... confess the crucified Jesus as their Lord and Christ, [and yet they...] blaspheme the Creator of the Universe, ... the God of Abraham and Isaac and Jacob. The very identification mark of heresy is thus the blasphemia creatoris.... (Oskar Skarsaune, “Heresy and the Pastoral Epistles,” Themelios 20.1 (October 1994): 9-14 at 10-11.)
Should we perhaps not be surprised that Justin Martyr is also regarded by some scholars as clearly having rejected Paul as on par with Jesus' words? (See our link.) Thus when Justin wrote about blasphemy by those who claimed to be Christians, Justin may very well have been thinking of Paul, just as we must ask ourselves if we truly love God.
"Justin Martyr was born about the year A. D. 100, and was a native of Neapolis in Syria. (Apology, sec. I.) At the beginning of the reign of Antoninus Pius he fixed his abode in Rome, and afterwards wrote numerous works, principally devoted to the defence of Christians. (Cave's Life of Martyr, vol. 2, chap. 6.)" George Reber, The Christ of Paul (1876), ch. 8.