Hi Lindsay,
I can state as definitively as anything in this insane world of ours can be stated that the scene you described NEVER aired on Dark Shadows.
There was however a scene between Louis Edmonds and Lara Parker that started out somewhat the way you describe this one ...
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In 1840, Edmonds played the aged Daniel Collins. When Angelique arrives, calling herself Valerie Collins, Daniel remembers her as Angelique who was eventually found out to be the witch who caused all the trouble back in 1795. However, the way the scene is played in 1840, it is Angelique who traps Daniel in the drawing room ...
In general, DS seems to have avoided direct references to religion. Apart from crosses, and a character occasionally exclaiming some variant of "Dear God!" The one exception to this rule is with the various incarnations of Trask, and the Trask family in 1897. The depiction of religious hypocrisy in the Trask family caused some fundamentalist organizations to print pamphlets and leaflets condemning DS as "the Devil's playmate" or something along those lines, which I personally think is a HOOT, but that's another matter.
It was interesting (to continue this discussion just a little further) that when Barnabas and Angelique are fighting Laura Collins in 1897, they both use invocations and holy names from Ancient Egyptian religion, rather as Isis was called upon by the forces of good in the classic 1932 film The Mummy (please don't confuse it with the remake).
Hope you continue to enjoy Dark Shadows.
Gothick