In the 2021 census, 44% of Australians identified as some sort of Christian, a big drop from 61% only 10 years earlier, while "No Religion" jumped from 31% to 46%.
Meanwhile, participation in church is harder to gauge, with some sources suggesting over 20% and others far less, but it's not in steep decline. It is nominal Christianity that is falling fastest.
In the census, over-75s are 70% Christian while young adults and young children only 30%. In church attendance the relative gap is bigger.
Census Christians are 54% female; adult church attenders are 60% female.
NSW and Queensland are more Christian than the south-western half of the country. Toowoomba is Australia's bible belt and Sydney the most Christian of the major cities.
The denominations dropping fastest in the census were Anglican (the most nominal) and Uniting (with the oldest age profile). They lost a third of their share in a decade while Catholic lost a fifth.
In large cities, the inner suburbs tend to be non-religious and the outer suburbs more Christian.
Census Christians are less educated than average (reflecting the age profile). But church attenders are much more educated than the Australian community. To put it the other way, educated people are more likely to be either No Religion or a church attender, while less educated Australians are more likely to be a nominal Christian.
Migrants from non-English-speaking countries are more likely to attend church. If it weren't for lots of Christians immigrating, numbers in the churches would have fallen a lot this century.
Stuff about the census is from my analysis. Stuff about church attenders is from other sources, mainly NCLS Research.