Wiktionary
a. (alternative form of honest-to-God English) adv. 1 honest; truly, really; (non-gloss definition: used to emphasize that a statement is true.) 2 Truly, really.
Wikipedia
Honest to God is a book written by the Anglican Bishop of Woolwich John A.T. Robinson, criticising traditional Christian theology. It aroused a storm of controversy on its original publication by SCM Press in 1963. Robinson had already achieved notoriety by his defence of the publication of Lady Chatterley's Lover.
Robinson's own evaluation of Honest to God, found in his subsequent book Exploration into God (1967), stated that the chief contribution of this book was its successful synthesis of the work of seemingly opposed theologians Paul Tillich, Dietrich Bonhoeffer and Rudolf Bultmann.
Usage examples of "honest to god".
Admitting that you've got honest to God feelings is a crime, isn't it?
He honest to God wore a pocket protector and a band aid on his glasses.