'No true Christian could vote for Donald Trump.' 'Real Christians are pro-life.' 'You can't be a Christian and support gay marriage.' Assertive statements like these not only reflect growing religious polarization but also express the anxiety over religious identity that pervades modern American Christianity. To address this disquiet, conservative Christians have sought security and stability: whether by retrieving 'historic Christian' doctrines, reconceptualizing their faith as a distinct culture, or reinforcing a political vision of what it means to be a follower of God in a corrupt world. The result is a concerted effort 'Make Christianity Great Again': a religious project predating the corresponding political effort to 'Make America Great Again.' Part intellectual history, part nuanced argument for change, this timely book explores why the question of what defines Christianity has become, over the last century, so damagingly vexatious - and how believers might conceive of it differently in future.