Luke 6:43-45 (ESV)
"For no good tree bears bad fruit, nor again does a bad tree bear good fruit,  for each tree is known by its own fruit. For figs are not gathered from thornbushes, nor are grapes picked from a bramble bush.  The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.
Jesus turns the old adam on his head. Our sinful nature wants us to believe we can become good by doing good. And aside from man’s inability to really discern what is good, being a sinner as he is, even if we were able to discern good from bad, Jesus shows the illogic of the whole endeavor.
I say man has an inability to discern good. I shouldn’t say that it is completely hopeless. But for every sin I could possibly posit that today a westerner, even perhaps an easterner, would see as bad, I could give you historical examples of times when people thought they were good. We can talk about sexual taboos, not just the domain of religious freaks mind you. We could talk about muder, whole societies have been built on ritual murder. And when it comes to things we think are good, there are societies that have thought them bad. History itself exposes the fallacy that man can determine good from bad on his own. We are lucky if we can figure out what is good for us, much less what is good in general. Of course the reverse is also true. C.S. Lewis in mere Christianity can find examples of each one of the Ten Commandments being held up as a virtue in a pagan society, even finding a Viking to condemn adultery.
Bad trees just can’t produce good fruit. Bad consciences can’t produce good works. Bo Giertz calls it a piety of the law, works righteousness, this idea that we can make ourselves good by doing good works. The problem is, if we aren’t good, our works aren’t good. We can’t make up for our sin, by sinning. Out of a bad conscience comes evil works. The intent of the works themselves are evil. They mean to by pas the cross and the forgiveness of sins, wherein alone a conscience can be cleaned and delivered good. Works meant to earn righteousness are evil, they mean to scale the tower of Babel and lay siege to heaven. They are meant to tell God his ways are not good enough for us. They feed the old Adam who himself wants to decide what is good, what should be appreciated, and be his own God. These are evil works.
But in the cross of Christ, buried with him into death through the waters of baptism, we are cleaned and given a good conscience, resurrected with him to the glory of the father that we might walk in the newness of life, and there having now been made good, our consciences do good works, that flow out of the love of Christ for us, works of the Holy Spirit working through us.