God will save whomever He chooses to save. The Christian should proselytize not because he thinks he can change everybody; he should proselytize because the Gospel being shared is the ultimate act of love: because he thinks he can love everybody.
God's relationship with man does not work in a way in which man stumbles and then God has to drop what he is doing in order to lift him up; rather, man stumbles so that God can lift him up. Hence it is utterly impossible to truly diminish his glory.