Idolatry: Saints should not Intermarry with Those Who Practise
Source: TTT
Exodus 34:16
And when you take some of their daughters as brides for your sons, their daughters will prostitute themselves to their gods and cause your sons to do the same.
Deuteronomy 7:3
Do not intermarry with them. Do not give your daughters to their sons or take their daughters for your sons,