The west has always been secular. It held onto a soteriological strain which often visits religions the moment they reach a peak of ecclesiastical ornamentation. Christianity internalized the Jewish law from which it descended and, thus, became a new church of ecclesiastical ornamentation. Many are well acquainted with the pairing of Buddhism and secularism, for instance. Less popular is the notion that Christ's incarnation into a body which could die, rise and then leave man alone could only ever result, theologically, in secularism.
It didn't, however, simply result in people no longer believing in God and going about their merry ways.