Real sugar doesn't trigger a fullness alarm, particularly where drinks are concerned. Diet drinks will not make you feel full, and neither will sugar drinks. The difference is that diet drinks are often zero calorie, and much less immediately damaging in terms of spiking blood sugar.
Your priest may have a point that diet drinks are best avoided, but for T2 diabetics, the vast majority of diet drinks will at least be safer than their full sugar counterparts. I'd rather drink coke zero than regular coke, or organic orange juice for that matter.