I appaude your bringing this up before marriage, facing this after marriage could be devastating and when children are concerned even more so. It is hard when each person has their own beliefs, so using caution is advised. I would continually pray for her and stay true to your faith and God will do the rest. A lot of people get a bad taste in their mouth about Christianity because of some terrible experience they had in the past and so they embrace a different religion to satisfy their needs and one they are confortable with. So, be strong and dedicated to your faith and show her how awesome it is to be a Christian and just love her with all you have and let Christ's light shine through you.
Just my few thoughts.
What lies behind us and what lies before us are small matters compared to what lies within us.
--Ralph Waldo Emerson
Don't judge each day by the harvest you reap, but by the seeds you plant.
--Robert Louis Stevenson