In the sermon, Dan explains why it is that theological adoption is such an important concept in the Bible, in spite of the word "adoption" being used just 5 times in Scripture. Essentially, God's adoption of His people has a "marking" function in the history of redemption. In other words, adoption plays a key role from the beginning (before God even created the world) all the way to the end (when all of God's adopted children enjoy the full privileges of their adoption in the new heaven and new earth).
Particularly interesting is Dan's explanation of how adoption relates to the doctrines of the Trinity, the fall of Adam, the election and failure of Israel, and the rescuing work of Jesus Christ that accomplished what God's "other sons" failed to do. Dan points out that humans are created for sonship - to enjoy the love of God as sons, and to show the love of God in families. Because "sonship" is so important in God's creative purpose, it is a travesty that there are orphans in the world - a mark of how damaged the creation is by sin.
A very important point (and one that I don't recall Dan having talked about on the blog before) comes in during Dan's conclusion. He says that when Christians adopt, they must not have the mindset of taking on a "project." Adoption is not a "project"; instead, it is an expression of deep, abiding, and profound love. Adoption is about relationship: what an orphan needs most is a relationship with father and mother. I think that this way of thinking about adoption is very helpful to prospective adoptive parents who are sorting through their motivations and developing a mindset about adoption.
Of course, there's so much more in the sermon, and I encourage you to listen to it. It's rich and Christ-centered.