What is the quintessential doctrine that all of Christianity is based on?

I really wished you could do that. But it isn't so. There are so many variants of Christianity that it is up to a founding interpretation of a denomination by particular persons, depending on which one you are referring to. Christ set it up, according to my interpretation, as the fulfillment of many prophesies concerning him from the Old Testament, who showed himself to be the man of God, "God is with us", who gave his life to save the rest of us.  All those post-Christ doctrines are suspect as to changing that in some way. I'm supposed to recognize Christ as the way to make peace with God, while some denominations require works for salvation. It ends up that for you to read the Bible and recognize what the truth is, or believe by hearing the word of God, you can respond in faith (trust) in Christ and in the promises of God. Those are indications that God considers you one of his own; not belonging to the world any longer.

John 3

18 "Whoever believes (trusts in; to have confidence in) in him is not condemned, but whoever does not believe stands condemned already because they have not believed in the name of God’s one and only Son."

Christianity is based on fabricated stories by the Roman writers of the NT, all based on a fabricated character they named Jesus, and the lies which they believe are prophecies for Jesus; none of which are true and every one of those claims are taken from the NT, and none are true and none are about a Jesus.

"Christ set it up, according to my interpretation, as the fulfillment of many prophesies concerning him from the Old Testament, who showed himself to be the man of God"   You surely have not read well those so-called prophecies which the NT writers want you to believe ans are taken from the NT and  which not one of them is about the Jesus character.  All such prophecies are lies, which proliferate the false claims about a false character in that religion which tells you that you must believe or die and go to hell. I challenge any preacher, priest, that pope man or any Christian to provide evidence for those so-called prophecies.  Until they do I will continue to call every one of them nothing but  lies used to promote a fictional character as being real.

The NT is nothing but a made up story, used to attempt the Sicarii to stop spreading the false message that a Messiah was coming to help them fight the  Romans;  he never arrived, and the destruction of Jerusalem is the evidence.  The NT was used  by Titus to deify his father Emperor Vespasian.  Therefore, Titus is the Jesus in the story. Trying to claim Romans 1:1-6 & 16-17 & 8:5-10 is futile

 as the Romans wrote not only Romans but the entire NT. Religion is a virus that uses believers to attack innocent people; they prey upon little children who cannot defend themselves. wake up and become honest.

Paul said "Christ Crucified.----Meaning God Sent Jesus down to sacrifice his life to pay the KARMA (sin debt) of humanity so they would not go to "the realm of the dead" (Hell) when they died, but rather, go to the "god plane" and "Live with him" 

Following the laws which are in the Bible of God and his son Jesus Christ.

Christian Sinner -- when did you wish that?

King James Version Bible

2 John Chapter 1

3. Grace be with you, mercy, [and] peace, from God the Father, and from the Lord Jesus Christ, the Son of the Father, in truth and love.

Mark Chapter 12

29. And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord:

30. And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment.

31. And the second is like, namely this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these.

If Jesus of Nazareth never died sacrificially, nor was resurrected on the third day as proof of Him being the sinless Son of God, then there would have been no Christianity. The tiny group of disciples would have hidden in the shadows, ashamed and defeated. So it was His death and resurrection that Christianity is based upon, for that was the gospel message that exploded on to the world's scene. Romans 1:1-6 & 16-17 & 8:5-10

Love one another.

The word of God, i.e. the Bible.

It should be the teachings of Jesus but we all know that isn't the case. At least not with the majority of denominations.

Be Christian.  Indoctrinate your kids to be Christian.  Encourage other people to be Christian.

That's pretty much the whole thing.  There's window dressing about morals, but when you actually look, the morality isn't all that moral and they don't apply it to themselves.  You can be as bad as you want and remain a hero to Christians...many of their most famous have done so with no consequences.

What's important is spreading the faith like a virus.  That's all that counts.