Early worship and admittance into God's presence
To fully understand Psalm 15 we need to look at early worship among the Hebrews and their neighbouring tribes. There were many local shrines, and with many travellers passing by the sacred space would be marked out, so that people did not accidentally stray onto it. Visitors would probably ask what were the requirements too worship at the shrine, and the answer from the priest might come back "You should have refrained from this...." or they might say "If you make the proper offering."
The Hebrews however, came to understand that the Lord required more than these basic things. Slowly they began to realise that He was calling them to live a moral life. It was less a bargain with God, and more that they needed to live a holy life, for God is holy.
Look up Micah 6.6-8. If needed, use http://biblegateway.com