God's faithfulness. Because he is God, whatever he is faithful in will be the strongest, most durable thing we've ever seen. So what is God faithful with? His mercy. God's steadfast love and mercy are rampant even in the Old Testament, and it blows us away. Israel has played the part of the unfaithful prostitute, forsaking God for man-made trinkets. God disciplined them, but for some inexplicable reason went outside his covenant, bringing Israel back to him; not because of what they've done, but because of the Lord's steadfast love, which keeps enduring for all of time. Hundreds of years of idolatry/adultery - hundreds of years of saying, "How can I worship what I can't see?" Hundreds of years of saying, "Look at what I've made! Look at what WE'VE done, growing in prosperity and power because of our own strength."
God exiled his people because of this and Israel was dispersed throughout Assyria and would never return to the Promised Land. Judah, however, the barely slightly more righteous half of the kingdom, remained intact even in Babylon. The Lord had prophesied dozens of times about keeping David's line alive and that he would raise the tent of David and restore Judah.
But even before that fully happened, he brought Judah back to the Promised Land! Things were different and though the temple, the house of the Lord, was built, his glory did not fill it. He sent prophets and spoke through them, but you can't help but deny that though this return had been great and God-done, something greater was yet to come. More on that another day, but before You-Know-Who comes, there is still a lot of mercy in how the Lord deals with returned exiles. As Hosea prophesied, he bought the unfaithful prostitute of Judah back, redeemed her and went beyond his covenant to have fellowship with and bless his people who really had done nothing worthy of such acts. He rescued them from being in the possession of another nation, and protected them as they rebuilt the wall and the temple.
In a lot of ways, this is a sign of - a step towards, if you will - future redemption. That redemption is in the past for us, but like I said, more on that another day.