This Sunday, regardless of whether your congregation is reading the RCL Track 2 lesson from Ezekiel, do yourself a favor and read all of Ezekiel 17 before you get to church on Sunday. I recommend that you read the CEB, which helps clarify some of the pronouns that get confusing. Here's a side-by-side comparison of the NRSV and CEB that you can read in 3-5 minutes.
It's the end of the chapter--verses 22-24--that are the reading for Sunday, but the rest of the chapter not only adds context to those verses but helps us understand what Jesus means when he talks about a mustard plant becoming big enough for birds of the air to nest in its shade. And, more than that, the first part of the chapter helps us understand why Jesus may have told the first parable in the gospel lesson--that of the farmer who scatters seed and does not know how it grows--and, perhaps most importantly, why Mark sandwiches the two parables together.
The first parable portrays the kingdom of God as something that comes up from the ground despite the farmer's lack of understanding. God is the one who gives the growth. The second parable portrays the kingdom of God as something that starts small yet becomes big enough to provide shelter for the birds of the air. And Ezekiel 17 portrays God's salvation as something that will not come through human intervention but through divine intervention--like a God-planted cedar sprig that will grow up into a mighty tree that is big enough for all the birds of the air to make nests in its branches.
In other words, not only is God's kingdom surprisingly big enough to shelter all, but it's also God's work, not ours. We don't make the kingdom happen. Our hope is in God.
Read the bits in Ezekiel about the Jerusalem king who was taken off into Babylon, who made an oath with his captor yet sent ambassadors to Egypt to ask for help. His efforts are described by the prophet not as a mighty cedar but as a vine that can be plucked up without much effort. God is the one who plants the cedar. God is the one who provides real security.
Sunday's parables give us glimpses of God's kingdom as Jesus understood it. Reading Ezekiel 17 helps us understand those glimpses even more fully.