Essential Doctrines of Christianity

The Bible itself reveals those doctrines that are essential to the Christian faith. They are 1) the Deity of Christ, 2) Salvation by Grace, and 3) Resurrection of Christ, and 4) the gospel. These are the doctrines the Bible says are necessary. Though there are many other important doctrines, these four are the only ones that are declared by Scripture to be essential. A non-regenerate person, or a cultist (i.e., Mormon or Jehovah's Witness), will deny one or more of these essential doctrines. Please note that there are other derivative doctrines of scripture that become necessary also, the Trinity being one.

The Deity of Christ
Jesus is God in flesh (John 8:58 with Exodus 3:14). See also John 1:1,14; 10:30-33; 20:28; Col. 2:9; Phil. 2:5-8; Heb. 1:8
1 John 4:2-3: "This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the Antichrist, which you have heard is coming and even now is already in the world."
The above verse needs to be cross referenced with John 1:1,14 (also written by John) where he states that the Word was God and the Word became flesh.
1 John 4:2-3 is saying that if you deny that Jesus is God in flesh then you are of the spirit of Antichrist.
John 8:24, "I said, therefore, to you, that you will die in your sins. For if you do not believe that I am, you will die in your sins."
Jesus said here that if you do not believe "that I am" you will die in your sins. In Greek I am is 'ego eimi,' which means ‘I am.' These are the same words used in John 8:58 where Jesus says "...before Abraham was, I am." He was claiming the divine title by quoting Exodus 3:14 in the Greek Septuagint. (The Septuagint was the Hebrew Old Testament translated into Greek.)
Jesus is the proper object of faith
It is not simply enough to have faith. Faith is only as valid as what it is put in. You must put your faith in the proper object. Cults have false objects of faith; therefore, their faith is useless--no matter how sincere they are.
If you put your faith in a vacuum cleaner, then you will be in a lot of trouble on the day of judgment. You might have great faith, but so what? It is in something that can't save you.
The Doctrine of the deity of Christ includes:
The Trinity - There is one God who exists in three persons: The Father, the Son, and the Holy Spirit. They are all coeternal, and of the same nature.
Monotheism - There is only one God in all existence (Isaiah 43:10; 44:6,8; 45:5,14,18,21,22; 46:9; 47:8). Mormons believe that many gods exist though they serve and worship only one. Therefore, they are polytheists which excludes them from the camp of Christianity.
The Hypostatic Union - That Jesus is both God and man.
The sufficiency of the sacrifice of Christ - The sacrifice of Christ is completely sufficient to pay for the sins of the world
As God - Jesus must be God to be able to offer a sacrifice of value greater than that of a mere man.
He had to die for the sins of the world (1 John 2:2). Only God could do that.
As man - Jesus must be man to be able to be a sacrifice for man.
As a man He can be the mediator between God and man (1 Tim. 2:5).

Salvation by Grace
"For it is by grace you have been saved, through faith -- and this not from yourselves, it is the gift of God -- not by works, so that no one can boast" (Eph. 2:8-9, NIV).
"You who are trying to be justified by law have been alienated from Christ; you have fallen away from grace" (Gal. 5:4).
This verse and its context plainly teach that if you believe that you are saved by faith and works then you are not saved at all. This is a common error in the cults. Because they have a false Jesus, they have a false doctrine of salvation. (Read Rom. 3-5 and Gal. 3-5).
you cannot add to the work of God. Gal. 2:21 says, "I do not set aside the grace of God, for if righteousness could be gained through the law, Christ died for nothing!" (NIV)
"Therefore no one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin" (Rom. 3:20).
"However, to the man who does not work but trusts God who justifies the wicked, his faith is credited as righteousness" (Rom. 4:5).
"Is the law, therefore, opposed to the promises of God? Absolutely not! For if a law had been given that could impart life, then righteousness would certainly have come by the law" (Gal. 3:21).

The Resurrection of Christ
"And if Christ has not been raised, our preaching is useless and so is your faith" (1 Cor. 15:14). "And if Christ has not been raised, your faith is futile; you are still in your sins" (1 Cor. 15:17).
To deny the physical resurrection is to deny Jesus' work, sacrifice, and our resurrection.
These verses clearly state that if you say that Jesus did not rise from the dead (in the same body He died in -- John 2:19-21), then your faith is useless.
The Gospel
"But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned! As we have already said, so now I say again: If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned!" (Gal. 1:8-9, NIV).
Verses 8 and 9 here in Galatians are a self declarative statement that you must believe the gospel. The gospel message which in its entirety is that Jesus is God in flesh, who died for sins, rose from the dead, and freely gives the gift of eternal life to those who believe.
Furthermore, it would not be possible to present the gospel properly without declaring that Jesus is God in flesh per John 1:1,14; 10:30-33; 20:28; Col. 2:9; Phil. 2:5-8; Heb. 1:8.
1 Cor. 15:1-4 defines what the gospel is: "Now, brothers, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain. For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures" (NIV).

Within these verses are the essentials: Christ is God in flesh (John 1:1,14; 10:30-33; 20:28; Col. 2:9); Salvation is received by faith (John 1:12; Rom. 10:9-10), therefore it is by grace; and the resurrection is mentioned in verse 4. Therefore, this gospel message automatically includes the essentials.