Goofs

Before Tanith enters the house, she is blood-spattered. After she goes inside, the blood spots have disappeared. Later we see them again.

When Rex arrives at the forest to witness the Black Mass, it is almost totally dark. Later in the evening when he phones de Richleau, however, the view outside de Richleau's window shows it is broad daylight.

When Rex and Tanith are riding in the green convertible sports car, there is a part where Mocata's face appears in the rear-view mirror and "communicates" with Tanith. In close-up, this mirror is shown to be mounted on the lower center of the windshield, but in subsequent shots from different angles, no such rear-view mirror can be seen anywhere on the center windshield.

From after Rex has fallen asleep at Marie's house (when he has removed his tie), his shirt alternates between having one and then two buttons undone in various shots. Notable is the final scene: when he is walking up the garden, there is only one button undone. When he enters the house, two buttons are undone.

When the Duke and Rex go back to Simon's house (when no one is at home) they suspect he might be up in the observatory. The door is seen half-opened in a close shot. It the next shot, as they approach it is seen to be wide open. The next shot also shows the door open all the way from the inside as they mount the staircase.

At the end of the film when the Duc is standing at the back door, he turns to go back into the house - and the film visibly jumps (as if a frame or two are missing).

In the observatory, when Simon lands on the head of the Goat of Mendes after being punched by de Richleau, it is clear the floor is made of foam rubber in order to cushion his fall.

When Nicholas lights the candle in the circle it is shown to be not lit. The camera pans down to show him pour the holy water and in the next shot, all candles are lit.

When Rex and Tanish are driving, the rear projection never changes angles when it cuts back and forth between their close-ups.

During the opening credits, a symbol is shown that is not Satanic, and it is also incorporated into the symbol on the priestly robes during the film. Inscribed within a Star of David, there is a six-winged seraph with the faces of a man, lion, ox and eagle based on the vision of Ezechiel. In Christian tradition, the four faces become associated with the four gospel writers: Matthew, Mark, Luke and John.

The film is set in the 1920s but when the Duc and Rex arrive at Simon's house and ring the bell it is a contemporary 1960s black plastic bell push.

The story is said to take place in April 1929, the green Bentley was not registered until September 11th 1931.

It's never established why Mocata is so hellbent on keeping Simon and Tanith in the cult. They're clearly unwilling participants, and the May Day celebration shows that there are literally dozens of willing Satanists in the area that are enjoying their worship instead. Since nothing was ever said about Simon and Tanith being special in any way, it's not clear why any replacement wouldn't do to restore Mocata's coven to 13 members.

Note: various cults have been known to kidnap unwilling members for all sorts of reasons (merely to expand the cult being one), but yes a specific reason should have been given in the script merely than an audience having to assume one of the many possible reasons.