All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
In principle, marrying a chaste Christian girl is permissible even if she is not ready to accept Islam.
No doubt, marrying her with the intention of saving her from disbelief and making her Muslim is a kind of calling towards Allah.
In spite of the above fact, you are not obliged to marry this girl and you do not commit sin if you leave her.
However, if you decide not to marry that girl you should keep yourself away from her. So, you should not look at her and should not stay alone with her since no man ever stays alone with a non-Mahram woman but Satan will be their third companion.
Allah knows best.