"While I deemphasize institutional metrics, more broadly conceiving the Church in mystical, sacramental, herald & servant conceptions, still, even mere institutional metrics don't reveal Christianity to be in decline per Gallup, Pew and other researchers."

Oh dear, such complacency.

Yes, the US is a couple of decades slow ... but the same early signs of secularisation that were evident in Europe some decades ago are evident for you. In all events denying the challenge is the surest way to ensure the success of secularisation.

And instead of pursuing just pure statistics, try dipping into experts who interpret them for underlying trends (e.g. Charles Taylor).