The first reading today would at first glance run completely contrary to yesterday's reading. On the surface it appears that God is about to unleash his wrath and Moses has to calm him down.
Here is where a basic principle of scriptural interpretation comes into play. To quote the Vatican II document on Sacred Scripture:
since Holy Scripture must be read and interpreted in the sacred spirit in which it was written, no less serious attention must be given to the content and unity of the whole of Scripture if the meaning of the sacred texts is to be correctly worked out. (Dei Verbum 12)
By unity we do not mean that there are no conflicts. There are clearly places where two authors report the same event in different ways. By unity what we means is quite simply, God does not change. The God of the Old Testament and the God of the New Testament are the same.
Take today's passage. Rather than seeing a God of wrath who had to be calmed down by Moses, it would be more consistent with our understanding of God to realize that God threatened wrath to see how Moses would react. Would Moses dare to speak up on behalf of the people? Would he show the qualities of a true leader? Much like Abram and the sacrifice of his son, it was merely a test.
Today's reading is a reminder that we must be particularly careful about the danger of taking a passage or worse yet a verse of the bible out of its context. But even beyond the context, we must always ask of some particular interpretation, how does this fit with our overall understanding of God and how God operates in the world?