At the end of the day, you should have a serious discussion with your partner.
Ask them about it. Find out why they believe in the practice and why they practice. Understanding and sympathy will go a long way in getting him/her to open up. Also find out the extend of the practice in your relationship.
If he/she does certain spiritual activities that you find disturbing, ask them to remove them from the house you share. Talki about what you can or cannot tolerate about the practice.
Decide if your relationship or marriage can survive a difference in belief systems. Depending on the extent of their involvement in witchcraft practices, you should decide whether you can tolerate it or not.
4. Relationship vision
A long-term relationship or commitment that includes children might cause a divide in how they are raised. How does his/her intend to incorporate the practice in long-term relationship goals.
5. Ending it
If your differences are too great, don't be afraid to break it off. At the end of the day, they did not open up to you about this side thus violating one of the basics of a relationship. Moreover, if they promise to stop practicing, remember old habits die hard. Especially ones connected to personal beliefs.