If you haven’t read Packer’s Fundamentalism and the Word of God, I’d suggest putting it on your “to read” list. Here’s a section from it that explains how God reveals himself in his acts (i.e. the Exodus) and also in speech (i.e. the Prophets) – and these go together, as Packer notes. Packer’s helpful argument also has to do with the necessity of Scripture.
“…According to Scripture, God reveals himself to men both by exercising power for them and by teaching truth to them. The two activities are not antithetical, but complementary. …Leave man to guess God’s mind and purpose, and he will guess wrong; he can know it only by being told it. Moreover, the whole purpose of God’s mighty acts is to bring man to know him by faith; and Scripture knows no foundation for faith but the spoken word of God, inviting our trust in him on the basis of what he has done for us. Where there is no word from God, faith cannot be. Therefore, verbal revelation – that is to say, propositional revelation, the disclosure by God of truths about himself – is no mere appendage to his redemptive activity, but a necessary part of it. This being so, the inspiring of an authoritative exposition of his redemptive acts in history ought to be seen as itself one of those redemptive acts, as necessary a link in the chain of his saving purposes as any of the events with which the exposition deals.”
“The need for verbal revelation appears most clearly when we consider the person and work of Christ. His life and death was the clearest and fullest revelation of God that ever was or could be made. Yet it could never have been understood without explanation. Whoever could have guessed, without being told, that the man Jesus was God incarnate, that he created the world in which he was crucified, that by dying a criminal’s death he put away the sins of mankind, and that now, though gone from our sight, he lives forever to bring penitent sinners to his Father? And who can come to faith in Christ if he knows none of this? No considerations could show more plainly the complete inability of man to ‘make do’ in his religion without a spoken word from God (p. 92).”