A Catholic proposal comes for a Pentecostal daughter. Or a Marthoma family likes a CSI boy. Both households are Christian, both are pleased, and everyone assumes the church part will sort itself out.
It usually does. It sorts itself out faster when these things are raised early, while goodwill is high and no hall has been booked.
Say it in the first conversation
Denomination should come up as plainly as job and hometown. Not as a problem to be managed, simply as a fact both sides are working with.
Families who leave it until the engagement discover that the questions below all arrive together, in a week when three hundred invitations are already printed.
Who conducts the wedding, and where
This is the first real decision. A Catholic party will normally be expected to marry in the Catholic church, before a priest and two witnesses. A Pentecostal congregation will expect its own pastor and often its own hall.
In practice many couples hold the service in one church and invite the other minister to take part, to read scripture, to pray over the couple, or to give the blessing. Ask both clergy early whether they are comfortable with that. Most are. Some are not, and you want to know which you are dealing with in month one.
The paperwork your church will ask for
A Catholic marrying a baptised Christian of another denomination needs permission from the Church for what canon law calls a mixed marriage. Your parish priest handles this. It is a routine application, not a rare exception, but it takes time and it is asked for in advance.
Expect a marriage preparation course as well. Most dioceses in Kerala and Tamil Nadu run them over a weekend or a few evenings, and the certificate is required before the banns are read. Many Pentecostal and CSI churches run their own counselling for couples and will ask you to attend that too.
Ask your own priest or pastor what your parish requires. Practice varies between dioceses and between congregations, and a neighbour's experience five years ago in another district is not a reliable guide.
Baptism, and the difference that matters
Catholic, Orthodox, Marthoma and CSI churches baptise infants. Pentecostal and most Brethren congregations baptise believers who are old enough to profess faith for themselves, and some will ask an adult from a paedobaptist background to be baptised again.
That last point causes real pain when it surfaces late, because to one family it is obedience and to the other it looks like the first baptism is being called worthless. Talk about it before the wedding. Talk about it again before the first child, and agree what you will do rather than leaving it to whichever grandmother is in the room.
Communion
In the Catholic Church, communion is normally for Catholics, so a Pentecostal spouse attending Mass will not receive. Most Protestant and Pentecostal churches practise an open table and will welcome anyone who professes faith.
Nobody is insulting anyone. Knowing it beforehand spares a spouse the moment of standing up at the wrong time in front of the whole congregation.
How Sunday will feel
A Mass follows a set order and is usually over in an hour. A Pentecostal service may run for three, with long worship, testimony and an open altar call. A CSI or Marthoma service sits somewhere between, with a liturgy and a strong preaching tradition.
Visit each other's church a few times before the wedding. Not to judge it, only so that neither of you is meeting it for the first time as a married person with in-laws watching.
The calendar
Lent is kept seriously in Catholic, Orthodox and Marthoma homes, with fasting and abstinence on particular days, and weddings are generally avoided during it. Feast days, parish days and Passion Week will shape the household calendar. Pentecostal families keep fasts too, often individually or as a congregation rather than by a fixed calendar.
Sit with both calendars once, early, and pick your date. It will save a difficult conversation with an uncle later.
Children
A Catholic entering a mixed marriage is asked to promise to do what they can to raise the children in the Catholic faith. The other party is informed of that promise. They are not required to make it themselves.
Decide together how that will actually work in your home. Which church on Sunday, whose Sunday school, who takes them. A promise kept by one parent while the other quietly works against it is worse for the children than an honest plan agreed at the start.
Take these to your priest or pastor
Bring a short list to both: what permission is needed and how long it takes, whether the other minister may take part, what preparation course is required, what is expected about baptism and about the children. Ask both sides the same questions and compare the answers in writing.
Clergy deal with this constantly. Most will be glad you came early, and the ones who raise difficulties are easier to satisfy in March than in November.
Endeavouring to keep the unity of the Spirit in the bond of peace. Ephesians 4:3, King James Version
Two families who worship differently and want the same thing for their children have far more in common than either has with the world outside. The differences are real and they are workable. They are worked out best in daylight, before anybody has an audience.