GST on religious and charitable institutions

We are a charitable and religious trust.  Of late we have decided to open a commercial complex with 5 to 6 shops. The revenue thus generated through rent will be utilized in the maintenance of the temple and trust. However we are currently not registered under GST. 

1.        IS it mandatory to register under GST

2.        Further to the rent accured from the shops in the complex, what is criteria for GST applied for shops?

3.        Any thresholds applicable for coming under GST ( 20 lakhs …)

4. Criteria for falling under GST exemption