In the footsteps of God, as the disciples of Jesus you are good to people from whom you will not receive anything good in return. God’s vertical disinterested love propagates this goodness. This verse says Christians do good unto those who have not done so for Christians, and of whom you should not expect them to do so. In this way you show the selflessness of heavenly love, and this is in distinction to “sinners,” namely those who live without wanting to know God.
33 And if you do good to those who do good to you, what benefit is that to you? For even sinners do the same.