PhD student Benjamin Field is accused of systematically targeting elderly and vulnerable people in order to become the beneficiary of their wills.
The son of a Baptist minister, Field is on trial alongside Martyn Smith, 32, for the murder of Peter Farquhar, 69, and conspiring to murder retired headmistress Ann Moore-Martin, 83.
Also in the dock is Field's brother Tom, 24 - a Cambridge University graduate - who faces a single charge of fraud relating to Ms Moore-Martin.
Field admits deceiving Mr Farquhar and Miss Moore-Martin, who were neighbours in the village of Maids Moreton in Buckinghamshire, into believing he was in a relationship with them both.
During cross-examination at Oxford Crown Court on Monday, Field admitted that he is not a Christian and had only put on a performance of being religious to help his fraud.
Oliver Saxby QC, prosecuting, asked Field if he had only become a church warden in order to meet elderly people he could defraud.
Field replied: "My motivation in going to church was about manipulating Peter."
Mr Saxby asked: "What faith promotes defrauding the elderly? Do you have any faith at all?"
"I believe in the idea of proof. I believe in the idea of some kind of a light in us. I wouldn't get more dogmatic than that," the defendant replied.
Mr Saxby said: "(Church) was a way of meeting potential targets. It's as simple as that."
Field said: "It's where I might meet people who were potential targets but first and foremost church was a way of impressing Peter."
The court heard that Field had also attended Ms Moore-Martin's Catholic church with her.
During the trial, the court has heard that Field is accused of gradually making Mr Farquhar and Miss Moore-Martin believe they were losing their minds.
His tactics included poisoning Mr Farquhar's food and drink with hallucinogens and neat alcohol, and writing "messages from God" on deeply religious Miss Moore-Martin's mirrors.
Prosecutors claim that Mr Farquhar - who was torn about his sexuality because of his religion - died from suffocation in October 2015 after undergoing a gay "betrothal" ceremony with Field.
Field had a sexual relationship with Miss Moore-Martin, bought her a sex toy and photographed her performing a sex act on him, the court heard.
The former church warden is accused of enlisting Smith - a magician - to help him make sure Miss Moore-Martin's death looked non-suspicious once she had changed her will.
Miss Moore-Martin died of natural causes in May 2017 before Field and Smith could carry out their alleged plan.
Mr Saxby accused Field of subjecting the pair to "years of torment".
"This is serious stuff. Lives wrecked and their families' affected," Mr Saxby said.
"More than affected - it's a really destructive thing that I've done," Field said.
Field has admitted defrauding Miss Moore-Martin of £4,000 to buy a car but denies recruiting his younger brother to con her out of £27,000 by claiming it was for a dialysis machine.
Police began investigating after Miss Moore-Martin suffered a seizure and was admitted to hospital where she confided in her niece about her relationship with Benjamin Fieldand the writing appearing on her mirrors.
A search of Field's home revealed copious notes about his victims as well as handbooks about planning suicide.
Following their arrest, Field and Smith were briefly held in a police van together where a recording device captured Field talking about "getting away with it".
Smith and Field deny charges of murder, conspiracy to murder and possession of an article for the use in fraud.
Field, of Wellingborough Road, Olney, Buckinghamshire, also denies an alternative charge of attempted murder but has admitted four charges of fraud and two of burglary.
One of the burglary charges relates to a third pensioner from whom three antique rifles were stolen.
In addition, Smith, of Penhalvean, Redruth, Cornwall, denies two charges of fraud and one of burglary.
Tom Field, also of Wellingborough Road, Olney, Buckinghamshire, denies a single charge of fraud.
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