The call of Jesus had been a call to ministry: “I will make you fishers of men” (Matthew 4:19). The Beatitudes picture the disciple both as receiving from God in pure passivity and as caught up into the motion of the God who acts and the Messiah who gives. The beggar can only receive, but he does receive; and the mercy which he receives makes him merciful. The peace which God gives him makes him a peacemaker. Men molded by the Messiah act in the world, so vigorously and so decisively that the world persecutes them for it.
In the metaphor of salt and light Jesus makes plain to His disciples how inseparable discipleship and activity are, how impossible any thought of a quietistic and contemplative discipleship is (Matthew 5:13-16). The disciples are salt and light by virtue of what the call of Jesus has given them and what the word of Jesus is giving them. They need not trouble themselves about how they may become salt and light, any more than a city set on a hilltop need concern itself about becoming conspicuous. Where they are and what they are, the fact that they are with Jesus and in communion with the Messiah, gives them inevitably a function which is as universal as the authority of the Messiah; they are the salt of the whole earth and the light of the whole world.
Both salt and light are, of course, thought of as having a salutary effect upon their surroundings. Salt seasons and preserves, and light dispels darkness and makes a man’s goings and comings certain and secure. But what Jesus is stressing in the metaphors is the fact that in salt and light nature and function are one; salt salts because it is salt, and light illumines because it is light. Salt which no longer salts has ceased to be salt. the disciple who ceases to minister has forfeited his existence as disciple and has destroyed himself. He has, by forgoing activity, disrupted his communion with the Christ; and there is no second way to saltness. A man can be light only by his communion with the Christ, and he can remain light only by shining.
The disciple is salt and light by faith; and faith is not chemical process but a personal relationship and therefore involves responsibility and obedience. The disciple cannot make himself light, but he can obscure his light. He cannot make himself salt, but he can in irresponsible disobedience frustrate his saltness. Jesus therefore implants with faith that holy fear which makes a man work in awe and trembling, lest he should have received the grace of God to no purpose. Again Jesus centers the disciple’s life squarely in God and puts it under the tension of the approaching end of days. The disciples live and work as sons of God, and they so live and work that God may at the last, when all false works are judged and all false glories have been erased, be glorified by all – be known as God, acknowledged as God, adored as God by His redeemed creation. (Matthew 5:16; cf. Philippians 2:11)
Martin Franzmann, “Follow Me: Discipleship According to Matthew”, pages 41-42