Who does he consider Christians? Many of the mainline denominations, such as the Episcopal Church, PCUSA, etc. are led by non-Christians who are agents of Satan. Those are the churches that are collapsing.
Consider the Independent Fundamental Baptists then. The flagship Bible college for the Baptist Bible Fellowship is Baptist Bible College in Springfield, Missouri. Look at its student enrollment, which is the reservoir of trust from which it obtains pastors for its congregations; its peak enrollment of over 2,600 in the 1970s had fallen to 390 by 2012.
Apparently its membership has declined as well, although the 2009 data looks faulty. I suspect the statistics reflect churn, splits, and Protestant relocation.