On Scanlon's expectational view of promises, a crucial part of why a promise is binding is that it creates an expectation of performance in the promisee. But if Sam vows something to God, then that doesn't create any expectation of performance on the part of God. For if Sam will perform the action, God has always known that. And if Sam won't, God's always known that. But if there were a God, one could make promises to him. So the expectational view is false.