The modern-day Court’s approach is to apply the three-part Lemon test to see if there is an Establishment Clause violation (that is, to see if the Jeffersonian “wall” has been knocked down).  The government action being challenged must:

(1)    Have a secular (non-religious) purpose;
(2)    Neither advance nor inhibit religion in its principal or primary effect;  AND
(3) Not foster an excessive entanglement with religion.

a.    Displays and Lemon: A creche display turns on its setting.  Taken together, do all the religious symbols sufficiently diffuse any endorsement of religion (be it Christian, Jewish, etc., since Lee held non-sectarianism is still religious)?  The Court looked at the locations of the display, its size, religious messages of or around the display, etc.
b.   Prayer and Lemon:  A non-sectarian prayer is still religious and must pass the Lemon test.