This passage occurs in the RCL "Pentecost"/"Ordinary"/"Proper" Season, Year B, most recently Summer of 2018.
Summary: When I first studied this passage for preaching, I was just finishing my first year of parish ministry. At that point, two things stood to me. First, the motif of "death and resurrection" in the first parable and the idea of "service to the neighbor" in the second parable. As I re-read this passage in 2012, I focused more on how this passage relates to congregational leadership and fostering faith. This year (2015) through, I propose that Jesus is the mustard seed that dies to become the tree.
Some words on church growth and leadership:
βαλη (from βαλλω; "thrown", 4:26) The most famous "sower" parable, which is found earlier in chapter 4, has a professional sower "sowing" (σπειρω) the seed. In this parable, we simply have a man throwing the seed. This reminds us that the sowers of the Word need not be simply authorized and trained clergy, but that God chooses the foolish and insignificant to do the work of the Kingdom!
Side note on Google: Part of Google's success as a company is their willingness to try things. They have created a culture where people are willing to throw stuff on the wall and see what sticks. In fact, when it comes to advertising, Google encourages companies to try as many permutations of their wording as possible to see what works. Churches tend to be much more cautious. These parables encourage us to try stuff without as much planning!
ελεγεν (from λεγω; "was speaking", 4:26) The imperfect tense is used here to portray Jesus speaking; this means that Jesus likely was repeating these parables more than once! Throughout this section, Jesus speaks in the imperfect tense, suggesting that he did not simply say this word but repeated it. In order for Jesus to get his message across, he needs to say it over and over. To go back to Google; Jesus has to try it in many ways to get it through!
αυτοματη ("automatically", 4:28) This is a humble reminder for all pastors that growth in the church is not a result of our own efforts, but the will of the Spirit, manifesting itself!
Some words on death and resurrection, as well as classic Lutheran themes:
καθευδη and εγειρηται (εγειρω) ("sleep and awake", 4:27): These words can also mean to die and to rise. This is a reminder that those of us that sow the seed will also experience death and resurrection. I know I have often felt crushed as a pastor by the inability of people to hear the word. And then risen to new life through worship and the Word! It also strongly suggests daily dying and rising to live out our vocation of sharing the Word.
χροτον...πληρυς σιτον ("grass...full grain"; "4:28") I am going to go out here on a limb, but I think this parable shows that sanctification and justification, while of the same movement, are not entirely the same. To be raised up (justified) does not suggest that God's work in our lives is done. The grass, while growing, must still grow into maturity. As χροτον (grass) it could still be eaten, but it will take time in order for it to become σιτον (seed itself) that could be used for next year's harvest. Similarly, we are reborn in Baptism and renewed in our weekly confession and forgiveness; God's Spirit still works on us, through this renewal, to transform and grow us, so that we might be of use to our neighbor. All metaphors are imperfect, but the emphasis here is not simply on the moment of receiving faith, but growing in the soil of the Word. As a confessional Lutheran, I would want to add that growth means more faith, which means simply becoming more dependent on God. To put this in a sound bite: the taller the plant...the more it needs it roots.
Some words I put together to think about Jesus Christ as the seed and the church as the plant:
καρποφορεω ("bear fruit", 4:28) The point of our dying and rising is to
bear fruit (Romans 7:4). In fact, one could argue that the seed that
is being sown in this case is not simply Scripture but Jesus Christ,
because the verb for the maturation of the seed is "παραδοι" from
paradidemi. This word means betray, which is a word that links and
moves the plot ahead in Mark's Gospel. Strangely, this is the only time
this verb appears with the word fruit; perhaps a further suggestion
that Mark is referring to Jesus as the word of God that dies for us to
become the tree.
αναβαινει ("ascend"; here meaning grow; 4:30) Jesus does not say that "once the plant has grown" he says, "growS and becomeS and makeS" all in the present tense. The growth of the mustard plant continues on and on. In this sense, I see the mustard plant (in the parable) as something supernatural; I offer it is the church, born by the death of Jesus Christ.
πετεινα του ουρανου ("bird"..."bird of heaven"; 4:32) The NET Bible suggests this phrase means "wild birds" as opposed to "domesticated birds." Even if the NET Bible overstates its case, a few points we can make if we compare the tree to the kingdom of God to the Christian community on earth, to finally, a congregation:
* The tree does not live for itself; the Christian life is not a life lived for oneself. This is true for an individual and for a congregation. (Vocation 101)
* To be the church is to host not simply nice people that "look like us" but all sorts of wild birds, maybe even ones harsh to the church!