Is the feeling of ease when treating a Christian maid kindly considered a fleeting thought or an act of apostasy?

Love and hate are of varying types. Loving a disbeliever for their religion or admiring their doctrinal or behavioral deviations is forbidden. However, loving them due to a work relationship, kinship, neighborly ties, or marriage is not forbidden. Evidence for this is the Prophet's (peace and blessings be upon him) love for his uncle Abu Talib, and the permissibility in Islamic law of marrying chaste women from the People of the Book.

Therefore, there is no harm in loving her due to her excellence in work, but you must hate her for her disbelief, while striving to guide her.