Most Anglo-Indian families are Christian, and there is an assumption that this settles the question. It settles less than people think — Catholic and Protestant households run differently, and practising and nominal are different again.
None of this is a problem. Unexamined assumptions about it are.
The questions that actually come up
Which church, weekly. If one family is Catholic and the other Anglican or Baptist, whose church becomes the family's? This is usually decided by drift rather than discussion, and the family that drifted away often minds more than they expected.
The wedding itself. Where it happens and who officiates is frequently the first real negotiation between two families. Better done early, calmly, than three weeks before the date.
Children. Baptism, first communion, confirmation, which school. Sort it before the wedding. It is a straightforward conversation between two people who agree, and a bitter one between two who assumed.
How observant. One person for whom Sunday is fixed and non-negotiable, and another for whom it is Christmas and Easter. Neither is wrong. They should know about each other.
Funerals and the end of life. Nobody wants to discuss it. Families who have not discussed it find out at the worst possible moment.
If one of you is marrying outside the faith
Increasingly common, and it works — but it needs actual conversation rather than goodwill.
The Catholic Church has a process for mixed marriages; ask the parish priest early rather than assuming, because timelines are involved. Other denominations vary.
What matters more than the paperwork is agreement on the children and on what happens at the big moments. Couples who have settled that are fine. Couples who left it as "we'll work it out" are the ones who have the argument later, usually in front of both families.
How to raise it
Early, once, without solemnity. *How does your family do Sunday?* tells you nearly everything and costs nothing to ask.
This is not a compatibility test. It is a set of practical arrangements that are easy to agree in advance and painful to negotiate afterwards.