I've heard mixed things here myself.
It's touched upon in the Dylan bio I mentioned in the Dylan thread.
Basically, by the mid-70s many in the rock scene were reacting to the various excesses of the late 60s and early 70s. The deaths of the famous, the near famous, and those known only to those in the industry had been piling up along with varying levels of personal and professional dissolution for many of the same who didn't die. It's only natural that many in the industry began looking around spiritually.
The Dylan bio mentions an evangelical church in southern California which had a huge number of rock & roll types in the congregation for a period. It's the church Dylan attended for a while and many others, like the Eagles, were members too.
Anyway, many folks toyed with fundamental Christianity of varying types, for varying periods, with varying levels of conviction, and with varying results. Dylan "converted" for a while, although that conversion seems to be wrapped up in his relationship with the woman who'd become his second wife and lasted roughly as long as that relationship did. Others converted similarly and then drifted away too.
The Dylan bio points to the huge similarities between the born again Christian fad which swept the industry in the late 70s and the eastern religions fad which did the same in the mid to late 60s. Everyone played around with the idea because it was hip, very few actually kept up any interest beyond a few years, and even fewer did so seriously.
What this all boils down to was that Lennon was curious, was exploring his spirituality, and no more became a born again Christian in the late 70s than he did a Buddhist after visiting India in 1968.