I don’t know how Christianity is being defined here. Are they talking about organized religion, that is official religious denominations? If so, membership here in the US I think is on the decline. Especially in the mainline Protestant churches and the Catholic Church both of which have been moving far to the Left in recent years. The Catholic Church is still recovering from the devastating impact of the pedophile-homosexual priest scandal which shattered the credibility of the clergy among millions of the laity. Further, here in the L.A. Archdiocese, the Church was saddled with an enormous $660 million settlement for the victims of abusive priests. I suspect this will take us many decades to recover from.
Politically I see little indication that even 50% of this country is Christian; they are only getting the number as high as they have because they are including pro-abortion, pro-”gay marriage” people in it (who just call themselves “Christian”).