"I hold any writer sufficiently justified who is himself in love with his theme"

Henry James is suggesting that an author needs to be enthusiastic about their work. He thinks that if an author loves their theme, then they are warranted in their writing. This suggests that they are warranted in their choice of topic and in the way they discuss it. It indicates that if a writer is passionate about their work, then they will have the ability to discuss it in a manner that is significant and appealing. It likewise suggests that if an author is enthusiastic about their work, then they will be able to convey their message in a manner that works and powerful. To put it simply, if a writer is in love with their style, then they will have the ability to write about it in a way that is both significant and efficient.