The ruling that came down from a grand jury in Texas last week was a surprise – jurors declined to prosecute Planned Parenthood, and instead indicted two people involved in the video sting that attempted to show the abortion provider was selling fetal body parts.

There was no surprise, however, about the firm that jumped to defend one of those indicted.

Sandra Merritt, the anti-abortion activist who posed as a biomedical research executive in order to gain access to the women’s health organization and film covert videos, will be represented in her case by Liberty Counsel.

Liberty Counsel is the same firm that hit the big time last summer when it defended Kim Davis, the elected Kentucky county clerk who cited “God’s authority” when she blocked marriage licenses for gay couples after the supreme court legalized same sex marriage in every state. When the district court ruled Davis in contempt and sent her to jail when she refused to back down, the story became a lightning rod in the fierce national debate around religious freedom, attracting media attention from around the world – just the kind of case that is red meat for Liberty Counsel.

The stridently evangelical law firm based in Orlando has made a name for itself by taking on high-profile legal battles aimed at “restoring the culture by advancing religious freedom”. From same-sex marriage to abortion rights and gay conversion therapy, Liberty Counsel is busier than ever these days, much to the delight of its founder, Mat Staver, who says the firm has quadrupled in size since he started in 1989.

“From the year 2000 to the present, we’ve grown exponentially,” Staver said. “Cases in the areas of abortion and human sexuality have become a lot more complex [...] and there are lots of new areas where we can find cases where people don’t even realize yet that there could be a constitutional argument.”

According to Staver, the firm employs only 10 lawyers as part of a small, permanent staff of 40, but draws on a pool of 600 to 700 affiliated lawyers spread across the US, who share the “Biblical values” at the heart of his firm and work on cases ad hoc, sometimes for fees paid by the head office, sometimes pro bono. Liberty Counsel represents all its clients pro bono and relies entirely on donations for its prosperity.

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Before he trained as a lawyer, Staver was a Seventh-Day Adventist pastor in Kentucky, subscribing to a branch of Protestantism that proclaims Jesus Christ is poised to return to earth imminently; he was also a prominent anti-abortion activist. His desire to attend law school, he said, stemmed from realizing that he could do more to advance the pro-life cause in court than in church.

He met his wife, Anita, while still a preacher. After they married and finished law school, they picked Orlando as their base to open a commercial law firm in 1987.

“I got tired of living in the cold. I really wanted to go back to Florida,” said Staver, who is a Florida native.

Two years later, he formed Liberty Counsel and closed down his commercial business so he and his wife could focus entirely on work driven by their “Christian passion”.

The firm made a splash with stunts such as threatening to sue a library for handing out playful “witchcraft certificates” to kids after a Harry Potter reading.

The stridently evangelical law firm based in Orlando has made a name for itself by taking on high-profile legal battles aimed at “restoring the culture by advancing religious freedom”. One of its clients was Kim Davis, pictured here with Mat Staver and Mike Huckabee. Photograph: Timothy D. Easley/AP

But its bread and butter cases focused on defending pickets outside abortion clinics and the principles of religious expression in public spaces, ranging from displays of the Ten Commandments to Christmas events. Then the Stavers found an urgent purpose when the tide began turning in favor of gay marriage in state after state and, ultimately, the nation.

Staver makes it clear that in many circumstances – mainly those involving his fervent opposition to abortion and homosexuality – he believes scripture is the highest legal authority in the country. But he doesn’t immediately fit the image of the Bible-thumping Christian extremist. Rather, he presents a calm, reasonable manner. He shares this temperament with his co-counsellors, and it serves them well inside the courtroom.

“I can’t say anything bad about them as lawyers,” said Daniel Canon, who faced two Liberty Counsel lawyers in court while representing several couples whom Kim Davis refused to marry. “They performed professionally – they were neat and normal. But I’m divorcing that from the substance of their arguments, which is dangerous and without legal precedent.”

The firm’s professional reputation aside, it still faces opposition. The Southern Poverty Law Center has labelled Liberty Counsel a hate group, based on its public statements online and on religious radio stations.

“It’s despicable. That’s the kind of rhetoric we do not need. It labels people. It attacks someone’s human dignity,” Staver said of SPLC’s move. He compared it to being categorized alongside skinhead gangs or the Ku Klux Klan.

The Human Rights Campaign publicly deplores the firm’s stance, pointing out, among many examples, that the firm described a workplace LGBT protection bill as “one of the most dangerous and discriminatory pieces of legislation in modern times”. And the American Civil Liberties Union is often on the other side of the courtroom, challenging Liberty Counsel on key cases.

One of Liberty Counsel’s newest causes is in mounting legal challenges to state bans on gay conversion therapy for children.

Staver told the Guardian that such bans are “harmful for the country” because teens who “don’t want to have feelings that conflict with their values” and whose parents “are at their wits’ end” should have access to such programs, which have been widely discredited.

And he asserted that being raised by a gay couple is detrimental to children’s wellbeing, based on “research and talking to people who have gone through this”.

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While the firm’s doctrine may turn off many mainstream Americans, it appeals strongly to a vital segment at the right wing of the religious and political spectrum, and the donations are pouring in.

Documents filed with the Internal Revenue Service show that Liberty Counsel is a non-profit, tax-exempt company that in 2013, the most recent year on public record, earned total revenue of $4.2m and had total expenses of $3.8m, leaving a comfortable surplus. Staver said Liberty Counsel received $8m in donations last year.

He declined to go into much detail but confirmed a Reuters report that the firm has received a total of $1.5m from fracking billionaire and radical Texas pastor Farris Wilks, who runs a church called the Assembly of Yahweh that decrees homosexuality a crime, all abortion murder and is part of a project called the Salt & Light Ministry Biblical Citizenship.

That project emanates from something called the Salt & Light Council, based in California and which, according to its website, encourages pastors to send volunteers for training in how to be politically active about the church’s sacred causes, especially restoring “the Bible’s relevance in the critical areas of government and citizenship”. The Salt & Light Council, according to the IRS records, is controlled by Liberty Counsel, and Staver acknowledged he is chairman of its board.

Farris Wilks and his family have given $15m to a Super Pac supporting Ted Cruz for president.

Liberty Counsel also has a Pac but hasn’t tilted toward a candidate yet. Staver is waiting for the Republican primary to play out and then is likely to spend on TV ads as the main race heats up, he said. He also predicted confidently that with the help of Liberty Counsel, Merritt, the anti-abortion activist, will beat the rap.

As a force to be reckoned with, Liberty Counsel is just getting started.

“The opportunities are extensive ... where I see injustice I can’t sit on the sidelines,” Staver said.