Sound Doctrine

Jude 1

 1 Jude, the servant of Jesus Christ, and brother of James, to them that are sanctified by God the Father, and preserved in Jesus Christ, and called:
 2 Mercy unto you, and peace, and love, be multiplied.
 3 Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints.
 4 For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.

The New Testament list of approved writers are the original disciples and Paul. Listed with them in fellowship is James and Jude along with Luke and Mark. It really is a rather short list of ordained writers.

Galatians 1

I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
 8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
 9 As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed.

Muhammad (Islam)
Joseph Smith (Mormons) Uses phrase "Another gospel of Jesus" LIE
The irony of the error is too much to stomach.
The Watch Tower Folk (Jehovah's Witnesses)
and many others

Are not on the approved list and the Bible expressly warns us about them and the things they would try to introduce into your lives as being accepted of God or a way to receive eternal life. Do not let the world tell you what is right, God's Word tells us how to deal with cults and erroneous teaching.

Doctrine must point to Jesus and His Greatness. If it does not lift up the Lord Jesus, it is a cult.

If any man teach otherwise, and consent not to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which is according to godliness;  He is proud, knowing nothing, but doting about questions and strifes of words, whereof cometh envy, strife, railings, evil surmisings,  Perverse disputings of men of corrupt minds, and destitute of the truth, supposing that gain is godliness: from such withdraw thyself.
I Timothy 6:3-5

Neither pray I for these alone, but for them also which shall believe on me through their word;
John 17:20
Jesus approves His disciples as those who are empowered to set forth doctrine.

And when Saul was come to Jerusalem, he assayed to join himself to the disciples: but they were all afraid of him, and believed not that he was a disciple.  But Barnabas took him, and brought him to the apostles, and declared unto them how he had seen the Lord in the way, and that he had spoken to him, and how he had preached boldly at Damascus in the name of Jesus.  And he was with them coming in and going out at Jerusalem.
Acts 9:26-28
Paul was added into the fold, many more examples exist in the book of Acts.

 Romans 16
16 Salute one another with an holy kiss. The churches of Christ salute you.
 17 Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them.
 18 For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple.
To mark means to point out and identify.

Just like a war, the planes are marked with a friend or foe tag. We are in a war and the enemy (Satan) will try to deceive many.

 Matthew 24
4 And Jesus answered and said unto them, Take heed that no man deceive you.
 5 For many shall come in my name, saying, I am Christ; and shall deceive many.

II John 1
 6 And this is love, that we walk after his commandments. This is the commandment, That, as ye have heard from the beginning, ye should walk in it.
 7 For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist.
 8 Look to yourselves, that we lose not those things which we have wrought, but that we receive a full reward.