Religious affiliation on a World wide scale, when compared to religious affiliation within the UK, seem to be on very different trends. According to the Pew Research Facility the religiously unaffiliated - referring to atheists, agnostics and other people who do not identify with a religion - are declining as a share of the world's overall population. However they also attest that, by 2100, it's distinctly possible that those who consider themselves to have no religion will outnumber the religious in the UK. In 2010 64.3% of UK residents were Christian, 4.8% Muslim and 1.4% Hindu, with 27.8% non-religious. By 2050 Pew estimates both Hindu and Muslim numbers will increase, to 2% and 11.3% respectively, however Christianity will decrease to 45.4%. Non-religious will increase to 38.9%. In fact, Europe as a whole, is expected to see a decrease, by something in the region of 100,000,000, of Christians by 2050. The Worldwide increase in religious affiliation by 2050 is attributed to the rampant growth in numbers of Muslims, facilitated by high birth rates, and high populations, in areas of high Muslim affiliation, such as western and northern Africa, the middle east, and the "stans" area of Asia (Afghanistan, Pakistan, Turkmenistan etc etc).
So, to summarise, overall religious numbers are on an upward trend, however this is primarily down to growth in the number of Muslims, predominantly in the 3rd World. The western world is seeing a decrease in the numbers of Christians, and an increase in the numbers of non-religious. It's entirely possible that, in 100 or so years time we'll see a western world which is more than 50% non-religious, and a third world which is 90%+ religious, with the vast majority being Muslim.