That's not relevant to the claim that Christianity is diminishing in relevance in the West, by which I mean western Europe and the anglophone countries. It may be gaining in popularity in parts of the world where life is becoming more uncertain, but wherever the fruits of humanism and the Enlightenment can be found and life is longer, safer, more comfortable, easier, etc., Christianity loses market share.
Here's the data from one of the first surveys showing significant declines, demonstrates the difference between absolute and relative numbers. It compares the numbers of people who self-identify as Christians in 1990 to 2008. As you can see, the number of people calling themselves Christian rose from about 151 million to about 173 million while the total population went from 175 million to 228 million, which was a relative decline for Christianity from 86.2% of the total to 76.0%:
American Religious Identification Survey
from
http://www.washingtonpost.com/wp-srv/metro/documents/aris030609.pdf
Adult population, total
1990: 175,440,000
2008: 228,182,000
Total Christian
1990: 151,225,000 (86.2%)
2008: 173,402,000 (76.0%)
None/ No religion, total
1990: 14,331,000 (8.2%)
2008: 34,169,000 (15.0%)
And there are more of these surveys from Gallop and Pew showing further progression of this trend. People self-identifying as Christians fell further from 76.0% to 70.6% by 2014
http://www.pewforum.org/2015/05/12/americas-changing-religious-landscape/
Here's a bit more of what we have:
This should be of no concern to any Christian whose religion is private. It shouldn't matter how many other Christians there are if there enough to congregate with, assuming he or she wants to do that. Either way, the Christian is free to study his Bible, pray, say grace, listen to sermons and sing hymns, marry and baptize in the religion, wear and display religious iconography of their bodies, bumper stickers, homes and businesses, etc..
That ought to be enough for any sincere supplicant. Those who want more, such as injecting school-led prayer or creationism back into public schools, or imposing their religious beliefs on others who resent them for it, will be disappointed as they find public religion and the majority status of Christianity dissipating, and see Christianity take its proper place in a secular society and eventually join every other religion in Western culture such as the Sikhs, Jains, and Druids, who are largely invisible to those not looking for them.