I'm still waiting for the explanation as to how anyone knoes what Millenials will do in the future, at the age range (40-60) when changes in religious participation are the most commonplace? One's twenties and thirties are not and have never been the prime years for religious participation, at least not in Europe/US from which we have the most data. So absent all else, decline in church attendance etc. is exactly what you would expect of any generation that happens to be passing through that stage of life.
The article explains it. There is a predictable pattern within prior generations where people become less religious in their 20's, but then return to religion when they get married and have kids. There is evidence that these events are causal factors. People used to feel that raising one's kids in the religious tradition they were raised in was important, or at least withing some religious tradition. I know many Boomers and GenXers that were not at all religious but suddenly baptized their kid and/or started going to Church once they had kids.
Millenials are now as old as 38 and the older one's have been tracked for 20 years. Many are now married with kids, and yet they have not shown the previously predictable pattern of returning to religion by this point. While some may still do so at a later age, given that having kids is a likely causal factor, there is good reason to infer that most of those who haven't done so after having kids and reaching their mid-30's won't do so later on. The article also says that Millenials were raised with less religion than prior generations. Thus, the pull to return to it may have weakened below a kind of threshold. Another thought, not in the article, is that much of the pressure to return to religion after kids comes from the grand parents and extended family. Many Gen Xers had "Silent Generation" parents who were raised very religious and as Grandparents put lot's of pressure on GenXers to raise their kid's with religion. But with every generation, the degree of religiosity of the grandparents get's weaker. Millenials have GenXer and Boomer parents who aren't putting as much pressure on Millenials to raise their kids with religion.
Another possible factor not mentioned in the article is that grandparents don't play as big a role anymore in how kids are raised. This could be partly due to physical distance (adults with young kids living farther from their parents than they used to), and part due to cultural changes in whether grandparents serve as major caregivers and how much influence young parents let their parents have.