When the supreme court makes a collective decision, are each judge required to explain why they voted the way they did?

It's not required, but most Justices will write an assenting or dissenting opinion (or concur with another judge's opinion), at least for significant cases.

No, not required

The majority side rights the decision,  the minority side may write a dissent if they desire.. Both the decision and dissent may be collective or individual 

No.  They are not required.  And the atheist and the Christian cannot leave their beliefs at the door of the courtroom.  This never happens.  We are who we are.

They have to be non partisan but won’t. If any vote based on faith they need to be removed.