Likewise, the parables of the mustard seed and leaven (Matt. 13:31–33) would have been understood negatively, spoken “to them” (v. 13) and interpreted apocalyptically, in light of “the end of the age” (v. 40). These two terse parables are simply negative teaching devices with a single player, akin to the parables of the rich fool (Luke 12:16–21), the barren fig tree (Luke 13:6–9), and the counting of the costs (Luke 14:28–33). Leaven was commonly understood as a bad thing (cf. Ex. 12:15–20; 34:25; Lev. 2:11; Matt. 16:6, 11–12; 1 Cor. 5:6–8; Gal. 5:9), and the allusion to Nebuchadnezzar concerning the mustard seed (Matt. 13:32; cf. Dan. 4:12) probably evoked highly negative emotions in Jesus’ hearers. Thus the leaven and the mustard seed most likely would have been associated with the preceding and following “weeds” (vv. 25, 38), which were destined to be “burned” (vv. 30, 40)—especially since the mustard seed and leaven parables are given no explanation (a point rarely appreciated). In this way, they simply communicate that God, in his great mercy, will allow wickedness to grow to its full measure (an idea seen throughout the Scriptures; cf. Gen. 15:16; Dan. 8:23; Zech. 5:5; Matt. 23:32; 1 Thess. 2:16) until the judgment at the end of the age. Again, if the mustard seed and leaven parables are bookended by a parable concerning God allowing evil to continue to maturity, then should we not assume the unexplained parables in the middle to communicate the same message? - John P. Harrigan, The Gospel of Christ Crucified, p.278-279