But the thing God will do is characterised by mourning: five times this Hebrew word occurs in as many verses. Families and peoples across the tribes will all be mourning. In v12 even the land will mourn. This is not just a quirky few in the corner, but it affects the whole of society. We see this in our own culture - just this week we had national mourning in response to another terrorist atrocity.
The mourning is sparked by someone being killed. Its a kind of 'oh my gosh, what have we done?' kind of response. Yet the identity of the person killed is obscure - clearly important, a leader or king, and in v10 a one and only son, so the most important for inheritance and pride of family. For ourselves reading this, we get a sense that it could somehow be God Himself.
Yet for the original readers that would not be possible. God is omnipotent, cannot conceivably suffer in this way. Talk to a devout Muslim today and you will have the same response - no way can this happen to God! Yet we can understand that this is God's bigger saving plan, to become human, to suffer and die at the hands of his own people, providing the way for them to be saved.
Yet return to v10: God first says 'I will pour out ...'. Grace and urge to call on God is given by God in the first place, and that enables the people to see their need for mourning, to see what they have done. In other words the order is:
God's grace → Desire for God → Mourning/Repentance
Our ability to turn to Him is God's initiative, He takes the first step. Our response means we can then receive from the fountain of God's provision (13:1), which cleanses us from our wrong. God provided water is a common theme - remember the woman at the well, as well as references in Ezekiel, Joel and Revelation. Interestingly cleansing comes here with no animal sacrifice necessary - God has already provided and catered for that.