Jacob explains that he was "compelled to write a 7-page (!) letter, to my brother, about it."
Although Jacob does not clarify which brother he wrote the letter to (his followers asked him in the comments but Jacob rarely answers questions posted to him since he stopped using his Ask.fm account nearly two years ago), many are making the reasonable assumption it is Jeremy Roloff.
Jeremy is the most outspoken Roloff on social media regarding his Christian faith, often tweeting and retweeting Christian pastors, websites and public speakers about how Christianity and Christians are under attack over issues such as Christian business owners not being allowed to refuse their services to gay people.
Here is part of what Jacob wrote on Instagram:
......For "Christianity has .. degenerated into a system of morality with only remote eschatological sanctions, and in practice is therefore a religion largely bereft of joy and power." But it's not too late to be saved, is his point of this book— because, "The Truth from which all the joy and power of Christianity proceeds [is] the truth of the Word made flesh—that the eternal life of God is given to man here and now in the 'flesh' of each moment's experience" and this understanding of the mystical experience of life is where Far Eastern religions greatly exceed our own.
Jacob publicly stating that he wrote a 7 page letter to someone in his family about a subject like religion is out of character for Jacob.
None of the Roloffs, Jacob and Jeremy included, have ever discussed each other's opinions or views publicly.
Back before Jacob stopped answering questions from his readers/followers, he once said on Ask.fm that he does not discuss religion with Jeremy because there is no point in starting an argument.
Jeremy (or his wife Audrey) has never publicly commented on Jacob's views on religion, even a couple of years ago when Jacob was more controversial in expressing his opinion such as when Jacob said the concept of "praying to a man in the sky" and thinking God is listening and granting or denying wishes is stupid and believing there is a Heaven and Hell is idiotic. Since that time though, Jacob's opinions on Christianity seemed to have mellowed.