This is a tricky one. We had an SDA at our place some time ago, but he was on permanent days, though he still had to be home before dark on Fridays, which was difficult in winter. What happened was that any time off that this guy had, he made up for on an hour-for-hour basis, and this seemed to work quite well. But as for having every Saturday off ........
The only way that I can think of is, say a shift of 4 on 3 off with the hours difference being made up in days off or extra days worked to make up the difference. or you could request that he takes leave for every Saturday that he wants off.
I don't know how discrimination laws work in Oz, but here in Blighty, you cannot discriminate against anyone, except smokers! But as an SDA, I doubt very much that he smokes!
The reason that I have posted a reply is that I too have faith in the Almighty, buy I am RC. So as an RC, I can go to church 364 days of the year if I so choose. Now, if I am rostered on for Sunday, I can go to Church any time I like to make up for it. But I have to say that I admire anyone who goes to Church, whatever faith they choose to follow.
Good luck with this one, and if practicle, let us know what happens as it may be of help to someone who reads these pages and finds themselves faced with a similar problem.