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Lomu, 37, who is fighting rejection of a transplanted kidney, was baptised into the Church of Jesus Christ of Latter-Day Saints in Wellington this year, the church has confirmed.
Since then he and wife Nadene Lomu have addressed congregations at "come and see" or "fireside" gatherings at a Mormon church in Mangere. They have appeared in on-line Mormon newsletters.
An article on the church's website described a meeting Lomu and wife attended. "The Lomus spoke about how their faith in Jesus Christ and membership in The Church of Jesus Christ of Latter-day Saints are helping them personally and as a family.
Lomu said that despite some of the choices of his youth, he always remembered what his mother had instilled in him, especially that he needed to pray to the Lord for guidance. He said that missionaries from the Church helped answer questions he had been asking for many years and that he and his wife felt that the gospel was right for them and their family."
Church spokesman Jared Viljoen said: "We welcome all visitors. Due to beliefs around family as a central focus to our beliefs and Christian values, lots of people are responding to that these days, which could have been what Jonah responded to."
Asked whether Lomu might pedal about Auckland spreading the church's message, Viljoen laughed. "No, that's probably not going to happen. That aspect of things is more for the young 18 to 35-year-olds, but later in life we also have senior couples going out."
Viljoen said another famous All Black great, Northland's Sid Going, had recently returned from an 18-month mission in Australia - so the Lomus could very well go on a mission one day too, just not by bike.
Lomu and his wife Nadene, whose maiden name was Quirk, were wed in Wellington last May by a "bishop from their church" according to an article in the Woman's Weekly at the time. Ma'a Nonu, whose father was a bishop in the Mormon Church, was one of the wedding guests.
It is not clear whether the family are giving any of their income to the church. Traditionally, members tithe 10 per cent of their income. A study by Reuters this year estimated that the church was pulling in US$7 billion ($8.5b) in tithes from its 14 million members each year.
Lomu, meanwhile, continues to have dialysis treatment in Auckland while awaiting a suitable kidney donor.
He had a kidney transplant in 2004 with an organ donated by his friend and radio host Grant Kereama. But in February Lomu revealed the kidney had stopped working and he needed another transplant.