Essentials of the faith would include these:
1) the full deity of Jesus Christ
2) monotheism (there is only one God)
3) the doctrine of the Trinity or Triune nature of God
4) justification by faith alone
5) authority and inspiration of Scripture
6) substitutionary atonement of Jesus Christ on the cross
7) original sin
8) virgin birth
9) bodily resurrection of Jesus Christ and believers
10) eternal reward of the righteous and eternal
punishment of the wicked at Christ's return
Non-essentials would be issues such as whether tongues are for today, or methods of church governance, etcetera.
Now, I know some would claim that this list is arbitrary, but I believe it is very plain that the above are not even questionable, and that anyone who denies them is operating outside of Christianity. Young believers may not understand the details of some of the points, but these items still define what the essence of Christianity is about. I would never claim that a person who is a younger believer is unsaved if they don't understand some of these points because they aren't very far in their growth process, but if someone is rebelliously teaching contrary to them, it's a good sign they are heretical or maybe not even saved.
I can't call something ice cream if it has no cream in it, and is a warm goopy mess. Same with Christianity. There are essential ingredients to Christianity, and if one's faith doesn't include certain core elements, you can't really call it Christianity.
Some cultic nuts would want to add other things, such as the Judaizing cult I belonged to (Armstrongites). Basically everything they taught was an essential, and they were the only true faith. I am sure that many Sabbathkeepers would claim Sabbathkeeping is an essential. Some claim speaking in tongues is an essential.