This spoilerless review comes from someone who in most cases prefers anime and doesn't enjoy manga in general. I have watched the short anime of Saint Oniisan and few years later decided to give the manga a chance. This is the only manga that I really love and enjoy reading!
I don't agree with opinions that this manga is not for religious people. I was raised as a Catholic and I am also interested in Buddhism and other religions. To me this manga almost belongs to spiritual literature. You can really appreciate and enjoy all the jokes in there when you are familiar with the biographies of Buddha and Jesus. The author must have thoroughly studied sources from both religions, sometimes also referencing Shinto and other spiritual teachings. It is a great reading even for people who are unfamiliar with either of them since the personalities of the main characters are just what you would expect from saints. There are also notes explaining some puns at the end of each episode. This manga shows Jesus and Buddha as normal guys, enjoying their vacation (taking a break from Heaven) in Japan, being nice and kind and...well saint! Their holiness also brings them to unexpected funny situations.
I believe that if those two met in real life they would be such good pals as in this manga. This also shows the deeper level or spirituality which is the same in both Christianity and Buddhism. This story is a great message of peace, not looking for differences but for things that those two religions are having in common.
The jokes are not offensive in any way and they are funny as hell...I mean heaven! ^_^" The art is very nice and the author also shows other characters connected to the main ones such as their disciples, angels and other heavenly beings, etc. The manga is episodic with only slight connection between the stories. This manga is simply cute!