Hi, I’m in the exact situation you’re referring to – baptized and confirmed Catholic, then kaboom, agnostic.
According to the Church, I must file a formal act of defection with my bishop in order to ‘leave’ the Church Militant; however, given that I am an apostate, I do not hold that the Church has any kind of authority, whether it be legal, moral, spiritual, or whatever, over me – and so I have not done so, and have no plans to.
Unless I
do file such paperwork, I am apparently considered Catholic – or at least I’m on all the mailing lists you can think of. Since I have not formally defected, the Church considers me merely ‘lapsed’; no matter what
I think, and I suppose I, or others in my situation, would not count as unbelievers as far as it is concerned as regards marriage or any other sacrament – the only thing standing between me and a return to the Church (as far as it is concerned) is ten minutes in the confessional and a few Hail Marys.
If you’re engaged to someone who has left the faith in the same manner as I did, I wish you all the best and hope that however you marry it is to your mutual satisfaction
