What is the main thing that Jesus taught? The answer from the Fourth Gospel (the most authoritative scripture) is that he brought the possibility of everlasting or eternal life; by which Jesus meant our immortal resurrection to life in Heaven.
In brief, the core of Christianity ought to be knowledge of Heaven, and the implications of this knowledge for this mortal life (including what we need to do if we want immortal Heavenly life for ourselves).
In support of this; I have collected the verses that specifically reference everlasting and eternal life in the Fourth Gospel.
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him.
But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life.
Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for him hath God the Father sealed.
And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.
Verily, verily, I say unto you, He that believeth on me hath everlasting life.
And I know that his commandment is life everlasting: whatsoever I speak therefore, even as the Father said unto me, so I speak.
That whosoever believeth in him should not perish, but have eternal life.
And he that reapeth receiveth wages, and gathereth fruit unto life eternal: that both he that soweth and he that reapeth may rejoice together.
Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.
Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day.
Then Simon Peter answered him, Lord, to whom shall we go? thou hast the words of eternal life.
And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.
He that loveth his life shall lose it; and he that hateth his life in this world shall keep it unto life eternal.
As thou hast given him power over all flesh, that he should give eternal life to as many as thou hast given him.
And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
The essential teaching is summarised in the final verse of Chapter 20, ending the Gospel proper: But these are
written, that ye might believe that Jesus is the Christ, the Son of God; and
that believing ye might have life through his name.