Your best possible course of action is to remain polite, compassionate, and kind. The best advertisement for the positive nature of your practice is your demeanor when faced with this criticism.
I too come from a non-supportive Christian family. Realize that you've discovered a wonderful opportunity in the Dhamma and try to have compassion for those who persist in delusion. Try and explain your practice if you can, but if they won't listen, then just go on your own and wait for them to see the change it brings in your life.
Gain and loss, status and disgrace,
censure and praise, pleasure and pain:
these conditions among human beings are inconstant,
impermanent, subject to change.
Knowing this, the wise person, mindful,
ponders these changing conditions.
Desirable things don’t charm the mind,
undesirable ones bring no resistance.
His welcoming and rebelling are scattered,
gone to their end,
do not exist.
- Lokavipatti Sutta