A very simple biblical answer: Eph 2:8-9 "For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— not by works, so that no one can boast."
I continue to be surprised by the enormity of what happened at the Cross. Sure, I understand that Jesus Christ left perfection, lived among us, and willingly submitted to being sacrificed for us. This alone is so big it's hard to appreciate how great a love this is and how easy salvation has been made for us. However, just recently Gamecock posted a thread by Steven Keller in which he points out how the civil state was separated from the religious order by the Cross because the penalty has been paid. The old order was compeletly turned upside down at the Cross. Yet today, just as in the Apostolic Era, the "wise" laugh at the simplicity of The Gospel.
We are so blessed beyond our understanding just because we have faith.