Law vs. Grace

This week again the story of Nadab and Abihu came up again in the reading in Gospel Worship.  It made me wonder again how we can apply this Old Testament idea of strange fire to a new covanant relationship with God.  I can't remember if we already discussed this earlier in the class.  How can we take the story of Nadab and Abihu who worked under the Mosaic covenant and expect God to treat us in the same way now that we are under Grace instead of the Law?