Is it necessary to believe all of the tenets of Christianity in order to go to Heaven? For example, if you believe in God as fitting the Christian image (at least roughly), but you don’t agree with stuff like original sin and the nature of sin itself, what happens?
I would think that the main thing would be believing in God and doing good works in his name, but IANA theologian, so I would like to know from people who are competent in this field (i.e., pretty much everyonbe on GD).
If there was a single answer to this there’d only be one religion. Which domination are you asking about? They all have their own opinions.
Even in a single denomination the answer isn’t easy. For example, I’m a Catholic, and in my denomination the answer depends on a few factors.
For one, if you die as a child too young to understand right and wrong, you go to heaven.
If you live somewhere where you’ve never heard of Christianity but you try to live a good and decent life, you’ll most likely still get into heaven.
If you’re a Protestant and follow all the basic tenants of your church, chances are you’ll get into heaven.
If you’re a Catholic, there are three things that are believed to be important for salvation. One, acceptance of Jesus Christ as your savior. Two, good deeds (James 2:26 “For as the body without the spirit is dead, so faith without works is dead also.”) and three, having communion (John 6:53 “Then Jesus said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you.”).
However in Catholicism it is also believed that nobody knows whether somebody will go to heaven or not. That’s for God to decide, since he is the only one who knows what’s really in somebody’s heart.
What you or I or anyone else believes is of no importance. What God expects and REQUIRES is what matters. That is the subject of a life long search for many.
Narrow is the way and straignt (small) is the gate that leads to life eternal and few there are who enter there.
Can you go somewhere that does not exist? 
But let’s say for argument it does. I was raised catholic, pretty rigid stuff, but it worked on an insentive system: You owe for every wrongdoing see? - lukily you can make up much of it after class… All you need to do is convert on your deathbed and, voila, you become Constantine the Great!
They do offer some wonderful parting gifts… 
In the OP, dwalin, you spoke of “all the tenets of Christianity”. As the other posters and myself have implied, there is no one “Christianity”, just denominational differences.For those who already profess Christianity I think it boils down to this. If you believe that God, in the Person of Jesus, came to Earth and lived as a human being, then died to redeem you from you from your sins, you will go to Heaven. Such belief is a gift, it comes from the Holy Spirit and can’t be earned by any actions on our part. Good deeds are performed out of gratitude for the gift, one might say, but have no bearing one one’s “eligibility” for Heaven.
Styles of services, rites performed, number of sacrements(if any) and so on, are irrelevant. Different people have different tastes in such matters, so they worship and behave in such a way that their thoughts are channeled into honoring God. I myself prefer a formal, liturgical service(heck, I AM an Episcopalian!), but it’s no better than a wide open charismatic revival.
Some years back there was a mini-series on television, Young Catherine, a rather fictionalized story of the early years of Catherine the Great, of Russia. But one segment impressed me as good writing. When she was worried about changing her faith to Russian Orthodoxy(it would be required of her), a priest quieted her misgivings in this way. I am quoting from memory her, as best I can He asked her “Do you know how a pearl is made?”, and when she said no he replied in this way. “A grain of sand finds it’s way into an oyster. The oyster finds this uncomfortable and begins coating it over with the material that forms the pearl. In the same way the words of our Lord can be uncomfortable, and people coat them over in different ways. But break open the pearl and inside you will find the same perfect grain of sand.”
I like that.