In fact, the United States has gone through many periods in which religious enthusiasm and affiliation waned, only to be renewed in a subsequent revival. Christianity is a rather durable faith. Many believers would ascribe this to the power of its truth claims, but its resilience also speaks to the adaptability of its core message.
But, yes, something is changing, and that change will strengthen rather than weaken the Christian church over the long run.
According to Dionne, the major change Christianity is currently experiencing is the divorce between conservative politics and conservative religion. The severing of this relationship will refocus and strengthen religion as a whole:
Religion is always corrupted when it gets too close to political power. It's possible to win a precinct caucus and lose your soul, to mistake political victory for salvation itself.
It is this approach to Christianity that is decidedly in decline, thank God, in part because conservative Christians themselves are rediscovering the Church's mission to the poor, the sick, the strangers and the outcasts. This augurs new life, not decay.