14 You are the light of the world. A city set on a mountain cannot be hidden. 15 Nor do they light a lamp and then put it under a bushel basket; it is set on a lampstand, where it gives light to all in the house. 16 Just so, your light must shine before others, that they may see your good deeds and glorify your heavenly Father.
Light, like salt, affects its environment by being distinctive. The disciple who is visibly different from other men will influence them. But the aim of his good works is not to parade his own virtue, but to direct attention to the God who inspired them. By so doing the disciple will give light to all (cf. Phil. 2:15).
Jesus is pre-eminently the light of the world (John 8:12), as Isaiah had prophesied of the Servant (Isa. 42:6; 49:6), but this role passed to his disciples (cf. Acts 13:47). The city set on a mountain reinforces the importance of being conspicuously different. As well the image of the city also echoes Isaiah (2:2-5; 42:6; and 49:6) in which Israel’s mission is to the world, to bring all nations to worship the one, true God – a task now given to the disciples. The church is like a candle, having been lit by God, not for the sake of the candle, but of the sake of the world. It needs to shine!
A bushel (grain measure of about 9 liters) put over an oil lamp would probably put it out, but the emphasis of the passage is on non-concealment (cf. Mark 4:21; Luke 8:16, ‘under a bed’). A secret disciple is no more use in the world than one who has lost his distinctiveness (v. 13). Your Father who is in heaven is a favorite expression in Matthew (cf. 5:45; 6:1, 9; 7:11; etc.), and reflects a major emphasis in Jesus’ teaching.
The admonition to let “… your light must shine before others, that they may see your good deeds…” (v.16) brings with it a certain tension. There are sources that will add comments to ensure it is understood that this is not “works salvation” – it is not necessary that one does actually does such. Yet they never address the tension that Jesus’ command has a certain directness about “must shine.” As Boring  notes, “The jagged edges of Jesus’ sayings should not be too quickly rounded off to make them consistent with other biblical teachings, or even with each other.” To which I would add, consistent with one’s already held theological suppositions.
Matthew 5:13-16 lives in tension with other parts of Matthew’s gospel. Where the very public display of belief and action is called out in our passage, consider passages such as 6:3-4, “But when you give alms, do not let your left hand know what your right is doing, so that your almsgiving may be secret.”
As Boring  points out, there are numerous such tensions in Matthew (e.g., 5:4 vs. 9:15; 5:9 vs. 10:34; 6:34 vs. 25:1–13; 8:12 vs. 13:38; 9:13 vs. 10:41; 16:6 vs. 23:3). Such tensions should not be pounced upon as examples of “contradictions in the Bible”; neither should they be too readily harmonized into a bland consistency, since they represent a dimension of wisdom teaching (cf. e.g., Prov 26:4–5). This is the nature of proverbial wisdom in general: “look before you leap,” but “he who hesitates is lost”; “fools rush in where angels fear to tread,” but “damn the torpedoes—full speed ahead!” The charge of inconsistency may be an attack on one’s integrity, but “consistency is the hobgoblin of small minds.” (Ralph Waldo Emmerson)
The “salt and light” sayings function as a warning to disciples not to fail in their mission. The salt and light sayings picture mission as inherent to discipleship, as saltiness is essential to salt and shining is to light. For salt, being salty is not optional. With these three metaphors of salt, light, and city, the Matthean Jesus strikes the death blow to all religion that is purely personal and private. Just as the sermon is heard not only by the disciples but also by the “crowds” (7:28–29), so also the church is not an esoteric community of initiates. The community that lives by the power of unostentatious prayer in the inner room (6:6) is not an introverted secret society shielding itself from the world, but is a city set on a hill whose authentic life cannot be concealed.
5:14 You are the light of the world. Light is a much more prominent and univocal image in the Bible than salt. Matthew 4:16 (citing Isa 9:2) has already associated light with Jesus and the Kingdom ministry in dark Galilee. Isaiah 42:6 speaks of Israel’s role in the world as a “light to the Gentiles” (cf. Isa 49:6; 51:4–5; Dan 12:3; Rom 2:19).
- Eugene Boring, The Gospel of Matthew in The New Interpreter’s Bible, VIII (Nashville, TN: Abingdon Press, 1994) 171-83
- New American Bible available at http://www.usccb.org/bible/books-of-the-bible/index.cfm (2017)
List of other Sources
- K. Beale and D. A. Carson, Commentary on the New Testament Use of the Old Testament (Grand Rapids, MI; Nottingham, UK: Baker Academic; Apollos, 2007) 20
- Warren Carter, Matthew and the Margins: A Sociopolitical and Religious Reading (Maryknoll, NY: Orbis Book, 2000) 130-43
- R.T. France, The Gospel of Matthew in the New International Commentary on the New Testament (Grand Rapids, MI: William B. Eerdman’s Publishing, 2007) 153-72
- R.T. France, Matthew: An Introduction and Commentary in the Tyndale New Testament Commentaries, Vol. 1, ed. Leon Morris (Downers Grove, IL: InterVarsity Press, 1989) 116-18
- Daniel J. Harrington, The Gospel of Matthew, vol. 1 of Sacra Pagina, ed. Daniel J. Harrington (Collegeville, MN: Liturgical Press, 1991) 76-85
- Daniel J. Harrington, “Matthew” in The Collegeville Bible Commentary, eds. Diane Bergant and Robert J. Karris (Collegeville, Minn.: Liturgical Press, 1989) 869-70
- Craig S. Keener, The Gospel of Matthew: A Socio-Rhetorical Commentary (Grand Rapids, MI: William B. Eerdman’s Publishing, 2009) 160-75
- Bruce J. Malina and Richard L. Rohrbaugh, Social-Science Commentary on the Synoptic Gospels (Minneapolis, MN: Augsburg Fortress, 2002) 39-51
- John P. Meier, Matthew, New Testament Message 3 (Collegeville, MN: Liturgical Press, 1990) 37-45
- John J. Pilch, The Cultural World of Jesus: Sunday by Sunday, Cycle A (Collegeville, MN: Liturgical Press, 1996) 28-30
- Mark Allan Powell, God With Us: A Pastoral Theology of Matthew’s Gospel (Minneapolis, MN: Augsburg Fortress, 1995) 122-38
- Turner and D.L. Bock, Matthew and Mark in the Cornerstone Biblical Commentary, vol. 11 (Carol Stream, IL: Tyndale House Publishers, 2005) 75-82