Mentsch1: Because Evangelical Christianity and Wahhabi Islam are fundamentally at terrible odds and contradictory with each other. And for a child to be pulled into those two extremely different directions by two differing parents is a terrible crime against the child’s well-being.
And since birth parents, in that case, effectively agreed to raise their children as evangelicals when they got married and before they had children, and in fact were raising them as such for a number of years, for one parent to unilaterally make a sudden change of the status quo is a wrong inflicted upon the children as well as upon the parental rights of the other parent and on the jointly agreed upon structure they entered into when they joined in union.
Your agreement, btw, is no different than most agreements where frum divorcing parents agree to continue raising their children frum post-divorce.