What is the Christian element in Frankenstein?

In arrogating to himself the right to create, Frankenstein may have become more god-like, but he's nothing like the God worshipped by Christians. For whereas the Christian God only creates out of love, Frankenstein is motivated by purely selfish reasons. That would explain why Frankenstein treats his Monster like an object, an object specifically created to serve him and bow down before him like a pagan god.
There's no sense whatsoever that Frankenstein has put any kind of love into creating the Monster; he put him together to satisfy his own needs, his own morbid scientific curiosity, his yearning for greatness, and so on. Frankenstein's hubris stands as a warning to those who would attempt to turn their back on God and create their own value system from scratch, taking self-realization as the only true human end.

There are numerous Christian allusions and allegories throughout Frankenstein.
One allegory is the creation of the monster itself. Frankenstein pushes the limits of his scientific knowledge further and further until he thinks that he understands how to create life. He begins to think of himself as almost god-like, with the ability to both create life and, later, to deny the creation of life. This story has similar roots in the Biblical creation of Adam, who also experienced a fall from his Creator’s graces. The contrast is that Adam falls from Grace because of his own disobedience. The monster falls from Frankenstein’s graces not because of anything he has done, but because of his creator’s prejudices. However, the monster does partake from his own Tree of Knowledge, much like Adam, as he reads Paradise Lost and begins to gain intelligence himself. Through this, he is able to see the ways Frankenstein has failed him and longs even more passionately for a family.
Justine also brings elements of Christianity into the novel. She is convinced that God is punishing her by continuing to take away people whom she loves. When she is accused of murdering William, a priest persuades her to falsely take responsibility for the crime to avoid going to Hell. Justine represents those who feel they can never do enough to earn God’s love and live in constant fear of a reckoning.