"Therefore, having this ministry by the mercy of God, we do not lose heart. But we have renounced disgraceful, underhanded ways. We refuse to practice cunning or to tamper with God's word, but by the open statement of the truth we would commend ourselves to everyone's conscience in the sight of God. And even if our gospel is veiled, it is veiled only to those who are perishing. In their case the god of this world has blinded the minds of the unbelievers, to keep them from seeing the light of the gospel of the glory of Christ, who is the image of God. For what we proclaim is not ourselves, but Jesus Christ as Lord, with ourselves as your servants for Jesus' sake. For God, who said, 'Let light shine out of darkness,' has shone in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ. But we have this treasure in jars of clay, to show that the surpassing power belongs to God and not to us." (2 Corinthians 4:1-7 ESV)
Some lessons on doing ministry:
1) While proclaiming the gospel, and teaching his Word, we will be prone to lose heart. We must not become discouraged at the difficulties but remember the mercy of God toward us -- him who gives us the inestimable privilege of declaring the good news about his Son.
2) We will be tempted to make the gospel more culturally acceptable, or engage in bait-and-switch, rather than speaking plainly about both the benefits and costs of being a Christian. We must have integrity in all our speech and claims.
3) People's unresponsiveness is not necessarily due to failure on our part. There is a spiritual opposition manifested in the veiling of minds against Jesus. Human depravity and spiritual blindness is real. This means that prayer becomes a most vital component of ministry.
4) We are not bringing people to see things our way, or to become our followers. We are not Lord -- Jesus is Lord. Our role is as servants to others for Jesus' sake.
5) New life is primarily seen in the opening of the heart and mind (hence, "light") to see the beauty and excellence of Jesus Christ as revealed in the gospel. Jesus himself is the glorious image of God.
6) Ministry will reveal that we are frail, fallible and very fragile. This should not surprise us, for the truth is that our frailty actually leads others to see the true surpassing value of God himself. We are not winning people to ourselves, but to the Lord. Again, as in #1 above, this becomes another reason for us not "to lose heart."