The Gospel message is simple, 1Cor. 15 1-4 "Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;And that he was buried, and that he rose again the third day according to the scriptures:"
Its the way we present it that can be make it effective or not.
The first time I witnessed to someone after being saved was when I was challanged by another high school freshman who say "I'm an athiest and have never had anyone tell convince me otherwise". I spent a very productive hour telling him the gospel, from creation, the fall, Christ's death and resurrection and our response to God's message. I didn't ask for a response and still wish I did. But at the time I was saved it was a personal thing I did when I was alone. Salvation was such a personal thing I didn't realize that it could be done with another person.
A friend is was missionary in Togo, West Africa, where there is little to no biblical education. He found that to most effectively present the gospelhe had to teach the people about the Bible, from creation, Adam, Noah, Abraham...etc. sort of like a sunday school class. After six months to a year of presentations, he would present the gospel. It was not uncommon for much of the tribe to accept God's salvation.