Christ being the Noun in the clause would govern the case of the preposition "in" (en). Christ is in the Greek Dative case. Please notice how the Dative case is understood below. Couple that with how we understand the Locative case in English. Do you see any similarities?The works were PREPARED for us before the casting down of the earth, right?
...and we are HIS WORKMANSHIP, CREATED in Christ Jesus unto GOOD WORKS
Just an aside unrelated question...do you hold the preposition "en" in the second quote to be locative or dative?
N-DSM
Part of Speech: Noun
Case: Dative (indirect object, "to"; also location "in", "at"; instrument, "with")
Number: Singular
Gender: Masculine