Religion isn't defined by monotheism. Hinduism, Buddhism (with its Bodhisattvas), Shintoism, Confucianism, Taoism -- all are polytheist. So there is no reason why the tennis religion should revert to monotheism.
That said, I would broadly agree with Prashant and Bhushan's fairly comprehensive criteria. If someone achieved all of those things, he would definitely be considered the all-time great. At the moment, Sampras falls short of undisputed all-time greatest status because he lacks a career Grand Slam.
If one takes Bhushan's argument about Laver into account (that he couldn't win more Slams during the 5 years of his career that he couldn't participate in Slams after turning pro), the same applies to Pancho Gonzalez (the greatest pro in the 1950s, who turned pro before really participating in too many majors!), Lew Hoad (the dominant player of 1956 with 3 of the Slams, who would certainly have won the Grand Slam 2-3 times had he played on as an amateur for the next 5 years), and his great rival in the early years Ken Rosewall (in my book, the greatest player of all time, who would certainly have won Wimbledon a few times had he remained an amateur longer -- he was runner-up four times, with a 21-year gap between the first and last, but missed 12 years of Slam tennis in his prime). Rosewall of course outlasted Hoad and even Laver (who was almost a decade younger!). He was still in the top-10 at the age of 41!
Agassi's game (like Jim Courier's) was ideally suited for clay courts, so it was not a surprise that he won the French Open (Americans had a mental block about winning the French after the 1950s, but that was shattered by Chang and Courier -- who paved the way for Agassi). The real surprise was that Agassi won Wimbledon once, but he got really lucky because there was absolutely no rain at Wimbledon in 1992, and the courts were unusually slow that year -- helping Agassi to pull off the near-impossible (for him) there.
Federer is now in line for his 7th career Slam title (he is in his 7th Slam final, and has won all his previous finals!). Rosewall and McEnroe, Connors, Laver, Borg and Sampras have more Slam titles (as indeed does Emerson, but his titles in the amateur era when he clearly wasn't even among the top 4 players in the world, don't count with me). But Roger definitely belongs in that company, and by the end of 2006 may belong near the top of it -- especially if he can win the French.
