I'm not so much a sportsy-follower and admit that I didn't know that Serena Williams is a Jehovah's Witness until she started talking about Barack Obama in this Slate piece, but I'm glad she did — this is fascinating stuff. Do you know why Jehovah's Witnesses tend not to vote? Passages like John 17:14 where Jesus says of those who follow him: "They are not of the world, just as I am not of the world." This other-worldliness is what Jehovah's Witnesses have interpreted as a directive to remain above terrestrial concerns like the election and neutral in all political matters.
Though voting isn't completely prohibited it's cautioned against — the Watchtower, the official publication of the Jehovah's Witnesses has suggested that whether to stop into the voting booth was one of personal conscience. Not limited to voting, Witnesses also don't serve in the military, pledge allegiance to the flag, or run for office.
The cautioning against politics does work — only 13 percent are registered to vote, and though they're the biggest religious group that opts out of voting, there are others, like the Amish and the Rastafarians (though even their attitudes have altered the last few years.) Is God a big reason to stay vote-free? In the US about 2 percent of people who don't register to vote cite religion as the reason.