So in setting out to define we cannot just be satisfied with the idea in 'simple' terms. Yes the gospel is or can be simple in approach and presentation. But it must become more than this.
This is as deep and wide as God. So it cannot become simplistic but must take on the essence of the very One who gave it to us.
A little background into how I started on this. For about the last 2 1/2 years I've been reading through the bible, specifically the NT by reading it over and over and over... I actually do about 7 chapters a day in one book and go until I get to the end. So for instance it would be Matthew 1-7 for a month, 7-14, etc...then on the first of each month I transition to the next section. This is a method that John MacArthur uses and one which I heard Robert Jeffries say, "it took God 1600 years to write the Bible, why do we want to read through it in a year?". Not at all meaning don't do it but why not take our time and I would have to agree with that assessment.
My approach being what it was I wanted to do the synoptic gospels in a row, Matthew, Mark, and Luke. Asking the main question, "What is the gospel?". In other words, what do the gospel writers, inspired by the Holy Spirit, say is the gospel? What is the core that makes everything function in its proper form and purpose? What did Jesus believe it was? What did the disciples believe it was?
This is what, after 10 months of reading, I can put into a sentence: "The Gospel is the grace of God, as revealed through Jesus Christ, for God, and to rebellious sinners." This is by no means flawless but it presents some ideas that I think we can be edified with and allow us to look deeper into the scope of God's great grace.
I'll look at some of the words and phrases more closely...next time.