At the very least it shows a lack of professionalism. I've got zero patience for excusing the bigotry of fundy christians, but I'm also not interested in proselytizing about that so I'm going to leave it be.
From a purely footballing perspective, writing on the armband shows a lack of professionalism and character. The best thing for the team in that situation is to just wear the armband and focus on winning the game, not creating a needless distraction by defacing the armband you are required to wear by the league. Maybe its not a huge impact, but it is an example of him putting his personal interests ahead of the team.
Add to that the fact that he's never looked like much of a world beater when we played Palace, or for England, and that they tried to fuck us around on the fee last time, and I am more than happy to find another centerback with less baggage. Maybe even one who is smart enough to keep his religious bigotry on the down-low like most people do.