Have you applied as yet?
If you are married before submitting the post ITA application, you will be treated as a married applicant (misrepresentation if you declare yourself as 'single') and your points are calculated differently.
Make sure your points after adding him to the application will be above the cutoff for the round for which you received ita. If not you can decline this invitation and wait for another, since they might reject if points go down.
If points are above you will have to produce marriage certificate, and his documents like, passport, medical, pcc and digital photo. Also note if he is claiming points for education, workex in Canada, he will have to produce those docs as well. Like eca, ref letter, degree etc.
No other issue with interfaith marriage.
I too have had interfaith marriage just as yours.