Here's a definition of the gospel or good news as spoken by Jesus Christ in person to the Nephite people in the Book of Mormon times (translated by the Prophet Joseph Smith):
8 And how be it
my church save it be called in my name? For if a church be called in Moses name then it be Moses church; or if it be called in the name of a man then it be the church of a man; but if it be called in my name then it is my church, if it so be that they are built upon my
gospel.
9 Verily I say unto you, that ye are built upon my
gospel; therefore ye shall call whatsoever things ye do call, in my name; therefore if ye call upon the Father, for the church, if it be in my name the Father will hear you;
10 And if it so be that the church is built upon my
gospel then will the Father show forth his own works in it.
11 But if it be not built upon my
gospel, and is built upon the works of men, or upon the works of the devil, verily I say unto you they have joy in their works for a season, and by and by the end cometh, and they are
hewn down and cast into the
fire, from whence there is no return.
12 For their works do
follow them, for it is because of their works that they are hewn down; therefore remember the things that I have told you.
13 Behold I have given unto you my
gospel, and this is the
gospel which I have given unto youthat I came into the world to do the
will of my Father, because my Father sent me.
14 And my Father sent me that I might be
lifted up upon the
cross; and after that I had been lifted up upon the
cross, that I might
draw all men unto me, that as I have been lifted up by men even so should men be lifted up by the Father, to stand before me, to be
judged of their works, whether they be good or whether they be evil
15 And for this cause have I been
lifted up; therefore, according to the power of the Father I will draw all men unto me, that they may be judged according to their
works.
16 And it shall come to pass, that whoso
repenteth and is baptized in my
name shall be filled; and if he
endureth to the end, behold, him will I hold guiltless before my Father at that day when I shall stand to judge the world.
17 And he that endureth not unto the end, the same is he that is also hewn down and cast into the fire, from whence they can no more return, because of the
justice of the Father.
18 And this is the word which he hath given unto the children of men. And for this cause he fulfilleth the words which he hath given, and he lieth not, but fulfilleth all his words.
19 And
no unclean thing can enter into his kingdom; therefore nothing entereth into his
rest save it be those who have
washed their garments in my blood, because of their faith, and the repentance of all their sins, and their faithfulness unto the end.
20 Now this is the commandment:
Repent, all ye ends of the earth, and come unto me and be
baptized in my name, that ye may be
sanctified by the reception of the Holy Ghost, that ye may stand
spotless before me at the last day.
21 Verily, verily, I say unto you, this is my
gospel; and ye know the things that ye mustb
do in my church; for the works which ye have seen me do that shall ye also do; for that which ye have seen me do even that shall ye do;
22 Therefore, if ye do these things blessed are ye, for ye shall be lifted up at the last day.