The person you spoke to at this wedding has used an illogical argument. This is not the type of argument that theologies generally make at a philosophical level.
But also you should know that God can be your money, property, children and/or your own ego or desire. That too is a type of theology taught within theology. Even an atheist can fall into that. As you addressed a Christian who would not believe in Brahma, it is a model of God each would reject. True. But that does not prove anything but the fact that each are rejecting a model of God. Same as an atheist.
Making the argument that a Christian rejects Brahma and vise versa is an attempt at a generalisation. It's used as if everyone is the same. You too, him too, thus both are in the same boat. Well, one of them could be right, thus in order to dismiss them one will have to explore each thoroughly. If not, it's also logically fallacious.
Cheers.