In this thread we post Bible verses where God promised something, one verse per post, in order how they appear in the Bible. 
I think this one would be first promise given:
Genesis 2:17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
I think this one would be first promise given:
Genesis 2:17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.