In the United States, Orthodox Christianity is by far the smallest and least-known of the three major branches of Christianity, representing about 1% of the population, compared with about 40% who are Protestant and 20% who are Catholic. Orthodox pews here have historically been occupied by immigrants from Ukraine, Greece and other countries with large Orthodox populations. Their American-born children often drift to other churches. But a homegrown Orthodox Christianity is strikingly emergent. Many of the young Americans new to the pews have been introduced to Orthodoxy by hard-edge influencers on YouTube and other social media platforms. Critics call the...