How is it we can even do a good work? John 3-4

John 3:21 (NASBStr)

But he who practices the truth comes to the Light, so that his deeds may be manifested as having been wrought in God.”

John, in this portion of his letter, is giving us the teaching of Jesus in regard to Nicodemus' desire to know "how to be born again."   Nicodemus was a teacher of the law and, genuinely, wanted to know who the "born again" teaching worked.   Jesus explains it to him and demonstrates that God is the person who gives the new brith.   This is a work of the Spirit, Jesus clearly teaches (John 3:8).   He doesn't want Nicodemus to think that his own works will somehow accomplish the rebirth.  It is an act of God.     He further explains this rebirth in the language of Jesus being the Light of the World.  As He switches metaphors, Nicodemus is told that those who are no reborn, walk in darkness.  Those that God gives life to are said to "be in the light."   And, having been put into the light by God they practice truth.   The real evidence of the believer walking in light and having been reborn is that they "practice truth."   That "practicing truth" is shown by the believers "deeds."    Again, Jesus wants Nicodemus, a strict follower of the Law, to remember, that anything good that man does is wrought by God.   Even the good deeds.   So, Jesus tells the teacher, Nicodemus, that these deeds are being manifested to the world around them to show they have been 'wrought in God."   Here is some great evidence that anything we do is, and must be, accomplished only because we are place in union with Christ.    We can't do good deeds unless God does the work.  Note the following commentator's words: 

Matthew Henry:  Here is the character of a good work:it is wrought in God, in union with Him by a covenanting faith, and in communion with Him by devout affections. Our works are then good, and will bear the test, when the will of God is the rule of them and the glory of God the end of them; when they are done in HIs strength, and for His sake, to Him, and not to men; and if, by the light of the gospel, it be manifest to us that our works are thus wrought, then shall we have rejoicing, Gal. 6:4; 2 Co. 1:12.

Any good work we do finds it's source in God's power, strength and doing.   When we claim the least amount of credit for it we steal from God and dishonor His Work in us.   Our part is faith.  His part is the production of work in our lives.   That is why it is called the "fruit of the Spirit" and not the "results of believer's work."