Nineteenth century America was very religious and scriptural. The tendency was to think of crises and tragedies in terms of Providence. You can look at Eugene Genovese's "A Consuming Fire" for examples.
Naturally, a more skeptical generation would find these scriptural references offensive. All the more so if they agreed with the other side. And there was much opportunity in Masters's day to exploit the change in sensibility to skewer those one disagreed with. But points scored against the rhetorical style of the day, don't particularly reflect on Lincoln personally. One might just as well find Lee's or Davis's or Jackson's references to Providence or God or the deity blasphemous or sacrilegious as Lincoln's. All Masters shows in his attack on Lincoln is his own disagreement with that President. Those who see the primary blame for the war resting on other shoulders won't be convinced by Masters's own rhetorical excesses.
Now there's a bold claim. Herndon, Lincoln's long-time partner who helped him midwife the Republican Party, not an authoritative source on Lincoln? Tell us why he isn't. Whom do you like better? Edwin Stanton? John Nicolay?
Come on. After Mary Lincoln, there was nobody in America who knew Lincoln better than William Herndon.