Only recently, the author had made it public, by posting on Instagram that he and his wife Shannon Bonne had decided to separate after 21 years of marriage.
In his latest revelatory Instagram post, Joshua said: "I have undergone a massive shift in regard to my faith in Jesus."
"The popular phrase for this is 'deconstruction,' the biblical phrase is 'falling away.' By all the measurements that I have for defining a Christian, I am not a Christian."
Harris added that he is open to "a different way to practice faith," but is "not there now."
Harris also apologised in his post for "self-righteousness, my fear-based approach to life, the teaching of my books, my views of women in the Church, and my approach to parenting."
These he has apologised for before, but he announced he had another regret.
Joshua wrote: "I specifically want to add to this list now: to the LBGTQ+ community, I want to say that I am sorry for the views that I taught in my books and as a pastor regarding sexuality.
"I regret standing against marriage equality, for not affirming you and your place in the church, and for any ways that my writing and speaking contributed to a culture of exclusion and bigotry."
Harris said: "This week I've received grace from Christians, atheists, evangelicals, exvangelicals, straight people, LGBTQ people, and everyone in-between," he said. But he added that there "have also been strong words of rebuke from religious people. While not always pleasant, I know they are seeking to love me."
In addition to writing a best-selling book, Joshua also served as senior pastor at Covenant Life Church an American megachurch in Gaithersburg, Maryland.
Stay up to date with the latest news stories from a Christian perspective. Sign up to our daily newsletter and receive more stories like this straight to your inbox every morning.