Θρησκεία = Religion
According to the Oxford dictionary ( https://www.lexico.com/en/definition/religion ), religion has the following two meanings :
1) The belief in and worship of a superhuman controlling power, especially a personal God or gods.
‘ideas about the relationship between science and religion’
‘...the role or power of religion and churches in society.’
2) A particular system of faith and worship.
‘the world's great religions’.
‘Hinduism is a complex religion, and there are many paths in it.’
"Θρήσκευμα" is a subset, it only has the second meaning (ie A particular system of faith and worship ).
I would not use "Τι είναι η θρησκεία σας;”. “θρησκεία” is female noun, so the correct way to ask is: "Ποια είναι η θρησκεία σας;”. You could also ask: "Ποιο είναι το θρήσκευμά σας;”