In the movie The Exorcist (1973), in the first part of it we see a priest that discovers profanations in a church.
Later in the movie, we was told that the profanations are linked to satanic rituals (and Regan's possession).
In the novel there's a part that is missing in the movie: the detective Kinderman analyzes the paint of the sculpture made by Regan for her mother and discovers that the same paint was used to make the profanations in the church and fingerprints of a "little hand (probably hand of a kid)" were found on a paper left by the guilty.
Did Regan commit profanations?
Those profanations were done before Regan shows signs of possession, so it makes me think that Regan was possessed before when we see it.