"All I did was write what I wrote, and then I prayed about it that he would, and that's it. There's nothing more to say."
Marjoram's book details his struggle with his sexuality and his life as a fundamentalist Christian for over 40 years.
In his email to Marjoram, Mr Robertson quoted and referred to parts of the Bible.
"We are not interested in your filthy lifestyle or book. The Bible says you are vile, strange (queer), reprobate, filth, sodomite, natural brute beasts, made to be taken and destroyed.
"I pray that you will commit suicide, you filthy fag."
According to its website, the Westcity Bible Baptist Church is a "family-oriented independent Bible-believing Baptist church".
Mr Robertson was born and raised in New Zealand. He is married and the couple have three young children, the website said.
Marjoram, who is originally from Sydney and was previously involved in Christian ex-gay groups, said the email was shocking but he wasn't too fazed about it.
"I guess he just sort of epitomises what a lot of fundamentalists and that type of really heavy religious people [have as] attitudes," he said.
"I realised it's exactly the sort of reason why I created my support group, because of people like him and those attitudes that...creep through the church."
Since Mr Robertson's email he had received messages of support from New Zealand and around the world.
"The internet's going crazy. I'm getting swamped from people all over the world saying they've posted on different LGBT [lesbian gay bisexual trans] sites and Christian sites in England, Canada, USA, Australia."
Marjoram said he had no plans to lay a formal complaint.
He said several congregations in Auckland, including St Matthew in the City and Ponsonby Baptist, had supported his book and support group.
The Human Rights Commission has condemned the email.
"New Zealanders who preach hate and violence are fundamentalists, they are extremists and they are the problem," said Human Rights Commissioner Richard Tankersley.
"Hatemongers who attack gay Kiwis need to know their behaviour has no place in Aotearoa.
"This is an appalling case and I know most New Zealanders will find it offensive and unacceptable" said Mr Tankersley.
"I encourage Kiwis to not be bystanders, to stand up if we witness hate attacks whether it's in a pub, in your church or even on social media. Let victims know they're not alone: but crucially let perpetrators know New Zealanders will not stand by silent while they spread hate and violence."
