They are not a peak body but this group has remarkable influence, writes Judith Ireland.
They are the lobby group that once said being gay was more hazardous than smoking.
That warns same-sex couples having children will lead to another stolen generation.
And that this week called for anti-discrimination laws to be "set-aside" during a plebiscite campaign so opponents of same-sex marriage could argue their case. 
This won't be the last you hear from the Australian Christian Lobby as the election year wears on. Not only is the ACL the most visible proponent of the "no" camp on same-sex marriage, they have a track record of attracting the biggest of political wigs - from John Howard to Bill Shorten - to speak at their events.
As Macquarie University expert on religion and politics, Professor Marion Maddox says, the ACL has achieved "remarkable influence with political leaders on both sides".
Yet they are not a peak body - and don't claim to be - and many church leaders insist the ACL does not speak for them.
So who are the Australian Christian Lobby?
The not-for-profit describes itself as "rigorously non-partisan," "non-denominational" and a "credible Christian voice in the corridors of power".
According to its website, the ACL wants a "compassionate, just and moral society" where the Christian faith is "reflected in the political life of the nation".
While its work opposing same-sex marriage has the highest profile, it has also recently campaigned against anti-protest zones around abortion clinics, the deregulation of brothels and euthanasia.
Based in Canberra with about 20 paid staff members, it was founded in 1995 by John Gagliardi, an evangelical Queensland businessman, and retired Baptist minister John McNicoll.
Its longest-serving public face has been retired SAS brigadier Jim Wallace, who is still involved as chairman of the board. The current managing director is Lyle Shelton, a former journalist, Toowoomba City councillor and Nationals state candidate (he left the party in 2007), who has also worked at his father's Pentacostal church.
Shelton says the ACL has about 50,000 supporters on its email list (there's no joining fee), who can be mobilised for campaigns.
"It's a similar model to GetUp!" he notes. [GetUp! says it has more than 1 million members].
According to the Australian Charities and Not-for-Profits Commission, the ACL is classified as a "large" organisation with a "revenue of $1 million or more".
Shelton says a lot of the funding comes from "mum and dad donors" giving small amounts, stressing there are no government or overseas donations. He declines to name any of the bigger donors for privacy reasons, explaining: "Unfortunately, they would be attacked by activists".
In 2012, coffee chain Gloria Jeans was faced with customer boycotts after it was revealed it had paid for advertising at a 2010 ACL election debate.
One Liberal source describes the group as "very well-regarded" across the board. In part, they attribute this to Wallace's standing as a former SAS officer, but also the ACL's "very respectful" and "not over the top" approach.
While suggesting that the ACL had a more receptive audience on the Coalition side (particularly post-Kevin Rudd), a Labor source said it had "pulling power" in Canberra and was able to get representatives of many denominations - from Coptic Christians to the Salvation Army - together all in one room.
While ACL has had some rocky periods in the corridors of power (Julia Gillard cancelled an appearance at their 2012 conference after Wallace talked about homosexuality being a health risk) it is credited with some major wins, like having Gillard make a pre-2010 election pledge to them not to change the definition of marriage.
ACL have already met with (relatively) new Prime Minister Malcolm Turnbull and have been directly consulted by Attorney-General George Brandis' office on the plebiscite.
The ACL may not be a "peak body" but it talks about "activating the Christian constituency".
And make no mistake about it, the ACL irks other parts of Australia's Christian community, who don't like the fact that the lobby group seems to speak for them.
Reverend Angus McLeay, who leads an Anglican church in inner-city Melbourne, laments that "the public, they just hear 'Christian' and they don't necessarily make fine distinctions".
Jesuit priest and human rights lawyer Frank Brennan said that while the ACL had a "place at the table", it did not speak for all Christians.
Melbourne Archbishop Philip Freier noted that as a church leader, he could go to Canberra of his own accord or combine forces through the National Council of Churches.
"I don't have a lot to do with the ACL."
When asked about the perception that because of their name, they were seen to represent a wider group than they do, Shelton said he "understood that concern".