This comes from an article by C.J. Mahaney called "The Legalist Within":
"How quickly we drift from this essential message [Jesus Christ crucified!]! We begin basing our relationship with God on our performance. We want to substitute our works — our Bible reading, our church attendance, our church participation — for Christ's finished work. We easily fall into the subtle but serious trap of legalism, because every one of us has a legalist lurking within."