Until then, Fred Phelps was known more for his civil rights work - and his nuisance lawsuits - than for his anti-gay activism.
But those who monitored Phelps say the focus on sexuality was always a part of his message.
"Phelps was obsessed with human sexuality for his entire life, going back six decades," says Mark Potok, a senior fellow at the Southern Poverty Law Center.
He notes that Phelps was profiled in Time magazine in 1952 because he had a street ministry against petting and dirty jokes.
It wasn't until 1991 that the modern movement began to form.
"Locally, the church is near a park in Topeka, Kansas, called Gage park," says Fred Mann, a reporter for the Wichita Eagle, who has written about Phelps in the past.
"They believed there was homosexual activity in the park. They went with signs, anti-gay signs and then it spread from there."
At first, the "God hates fags" slogan was nowhere to be found.
"I remember them talking about the beginning days of the picketing," says filmmaker K Ryan Jones.
His film Fallen From Grace followed Phelps and the Westboro Baptist Church for a year.
He recalls that they told him: "Originally our signs did just say 'gay' or 'homosexual' or that kind of thing, and then we started using 'God Hates Fags'."
They set out to shock - and they knew that those who weren't offended by the use of the word "fag" would bristle at the idea of God hating anyone, says Jones.
Their tactics seemed to work. Though "God hates fags" is not their only sign, it is one of the best-known and the one most associated with the church.
The signs also provided a pithy explanation of the church's theology, which is explained in greater depth on their website, GodHatesFags.com.
"'God loves everyone' is straight from the mind of Satan and his ministers that serve him," it says.
"In essence 'God loves everyone' means that man can lead a sinful life, violate the commandments of God daily, not fear Him and still go to heaven."
The site provides a detailed chart of everyone in the Bible it says have been killed by God's wrath.
The sharpness of their message combined with their antics at the protests created an intense reaction.
"There's a lot of people that do not like homosexuality, but they would never use that kind of language," says Mann.
"It was their behaviour on the picket line that appalled people. They didn't just show up with signs, they danced and sang. They were delighted to be at someone's funeral."
And the more the church members upset and outraged onlookers, the more validation they felt.
But they became much more notorious when they began in around 2005 to picket the funerals of soldiers who died in the Iraq war.
"That's when they went from being nationally known to globally known," says Potok, thanks in part to the bluntness of their message and the seeming mismatch between the signs and the targets of their protests.
"It's hard for people to understand," says Jones, "because they see signs that say things like 'Thank God for Dead Soldiers' and 'Thank God for 9/11'.
"And these soldiers weren't gay. Not everyone in 9/11 was gay.
"Any tragedy you can think of, they believe that is God's wrath on America because of our tolerance of homosexuality.
"It's evidence of the message they are trying to promote."
As the family and their message grew in prominence, so did the signs themselves.
At first, says Jones, the signs were a fairly do-it-yourself affair, made of magic markers and poster board.
Now, he says, they have an industrial printer, a professional laminating machine and an entire workshop devoted to making signs.
The Phelpses also proved adept at social media, creating Vine videos that attracted attention from news sites that would normally criticise the Westboro message.
In the end, their shock message was so well-known and their methods so loathed that they didn't even have to protest.
"At least half the time they would call up a place they were going to picket ahead of time," says Potok.
When news leaked that the Phelpses were going to be at Sandy Hook, for instance, it spurred international media attention - but the family never showed.
"They wouldn't bother," says Jones. "They already got what they wanted."
Though Phelps is dead, his church remains, with many more members to carry out his theology and carry his signs.
But the power of 'God Hates Fags' has become so strong that the signs - or even the protesters - are hardly necessary any more.