Michael Horton describes the simplest and most difficult truth of the Christian faith:
“Good News comes from outside of ourselves; it doesn’t well up from within. A report announces what has happened. Our natural tendency is to be the most certain of that which we know (or think we know) inside, deep down in our hearts. However, we must come to see that this is part of the captivity from which the Spirit releases us. The truth comes to us from the outside, turning us inside out. So the content and the form of delivery are inextricably linked. We do not accomplish the victory, but are the recipients of the report that it has been achieved. Salvation is not a program for us to follow; it is a gift to be received. That is the simplest and most difficult truth of the Christian faith.” (The Gospel-Driven Life, 110)
The Gospel is external truth. It is the happy news of the historical event of the life, death, and resurrection of Jesus Christ. The Gospel is news. It is not principles to live by. It is not your experience with God. It is not a “how-to” for life. It is not something you do. It is the news of something that has happened apart from you.
The Gospel is what God has done in the person and work of Jesus that is to be received by you. It is not accomplished by you, it is receiving what has been accomplished for you. This is difficult because self-righteously we want to play a significant part in the salvation of our souls, and it is simple because it’s God’s gift to us not our gift to God.