[Supreme Order (Admin): What do you think, Deus? Can you finish that sub-task?]
Allen asked Deus to handle that task since he was sure that it was within Deus' capabilities. Even if Deus couldn't finish that sub-task, at least Deus wouldn't just sit around and do nothing during the Main Task.
So even if Deus couldn't finish it, there would still be a considerable amount of progress during the completion of that sub-task. And besides, it wasn't like Allen wouldn't help in that sub-task as well since he didn't have anything to do. It was not like he would just wait for the Heretic Gods to appear at his doorstep and kill them.
Allen was certain that when he converted countless living beings into a believer of Zenchianism, that would be the time that those Heretic Gods would be alerted by their action and the Heretic Gods would likely eliminate them at that time.