Mr Justice Holman questioned whether Ankul Daga's oath would be correct.
The judge checked on the nature of Mr Daga's religious beliefs and whether he was aware what he was being asked to swear on.
Mr Daga said he was, explained the nature of Jainism and said he was happy to swear on a Bible.
Mr Justice Holman was overseeing a fight over money between Mr Daga and estranged wife Aparna Bangur at a public hearing in the Family Division of the High Court in London on Tuesday.
Mr Daga, who lives in London, wants Ms Bangur, daughter of Indian billionaire businessman Krishna Kumar Bangur, to hand him £2.5 million following the breakdown of their relationship. She disputes his claim.
The hearing is due to end later this week.
A BBC online guide to religions says Jainism is an "ancient religion from India that teaches that the way to liberation and bliss is to live lives of harmlessness and renunciation".
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