The US is becoming less Christian and less religious overall, according to a new study released by the Pew Research Center.

According to Pew, only 63% of the US population now describe themselves as "Christian," down from 75% just 10 years ago. Thirty percent say they are religiously "unaffiliated."

Washington state is one of the least religious states in the country, with only 45% of the population identifying with a religion.

King County is even less religious. Here a mere 35.07% say they are religious. Pierce County is only slightly more religious at 35.08%. The most religious county in the state is Garfield County, in the southeast corner of the state, where a whopping 60.1% of the people say they attend church services at least once a week.

Among the King County churchgoers, Roman Catholics make up the largest share, at 15.35%. Protestants of various denominations come next with 14.85%. Almost two percent are Jewish, 0.56% are Muslims, and 0.11% adhere to what are described as "Eastern religions."

Washington state is not the most irreligious state, however. That title goes to Massachusetts and New Hampshire, each of which has only 33% religiously affiliated. The most religious state in the country is Mississippi, where 63% of the population attend church services regularly.

Religion in the rest of the world
While the religious population in the US — and other developed countries — is declining, almost everyone else in the world is religious, Pew found. According to the latest edition of Pew's Religious Composition by Country, 2010-2050 report, some 7.7 billion out of 7.9 billion people in the world are affiliated with a religion.

This groundbreaking study considers the religious composition of 198 countries and how it is likely to change over almost four decades based on "differences in fertility rates and the size of youth populations among the world's major religions, as well as by people switching faith."

Based on Pew's forecast for 2020 for the number of members of each group, Christianity ranks first with 2.383 billion people, out of a global total of 7.7 billion religious people.

The largest group of Christians is in sub-Saharan Africa (650 million), followed by Latin America (585 million), Europe (535 million), Asia and the Pacific (320 million), and North America (277 million).

Among the countries with the largest Christian populations are the United States (259 million), Brazil (185 million), Mexico (118 million), the Philippines (102 million), Nigeria (96 million), China (72 million), Ethiopia (62 million), Italy (48 million), South Africa (43 million), Uganda (40 million), Tanzania (39 million), France (37 million), Spain (36 million), and India (34 million).

Methodology
Pew's report is based on several sources: