when Isa (AS) will come, Muslims who will NOT believe him will at that time be also a 'muslim ummah'. Because whoever will start believing in him will be considered non-muslim.
So really, if you think about it .. this Muslim Ummah that you and coolguylin keep talking about will always be there, who will be declaring anyone not agreeing with their beliefs as 'non-muslims'
YOU are not the one to decide that, Its bound to happen, and it WILL happen..