Doctrine is the core set of beliefs of a church or denomination. It is the fundamentals of the faith. It should be what separates you from others.
As we catapult towards the end of this dispensation it seems the ecumenical movement is growing in strength every year. Which according to prophecy is to be expected. After all, religion will be a unifying factor of rallying people to worship of the anti-Christ. As the religions of the world blur the differences in beliefs it is even more important for Christians to stand firmly on the doctrines of the faith and remaining faithful to the doctrines passed to us through the Word of God rightly divided. Let us look at what the Bible says about our doctrines.
1 Ti 3: All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
 That the man of God may be perfect, throughly furnished unto all good works.
1) The Bible is our one and only source of doctrine.
Knowing and applying the doctrines of the faith brings spiritual maturity (perfect)
Reproof- what is wrong
Correction- how to get it right
Instruction in righteousness- how to stay right
Ep 4:11] And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;
 For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ:
 Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:
 That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive;
 But speaking the truth in love, may grow up into him in all things, which is the head, even Christ:
2) Unity of the faith- common core set of beliefs. As we grow in Christ and the fundamentals of the faith become a way of life and not something we memorized we become more like Christ.
3) Changeable doctrine is a sign of spiritual immaturity.
A spiritually immature Christian can be swayed by different doctrines and “new” ideas. But the mature Christian is able to stand firm on the foundation.
2Tim.4: Preach the word; be instant in season, out of season; reprove, rebuke, exhort with all longsuffering and doctrine.
4) Doctrine is the means to reprove, rebuke and exhort.
Reproof- speak with disapproval
Rebuke- to blame or scold
Exhortation- to urge earnestly, to lift up
Tit.1: Holding fast the faithful word as he hath been taught, that he may be able by sound doctrine both to exhort and to convince the gainsayers.
5) Doctrine is used to exhort and to convince the gainsayers.
Tit.2: Young men likewise exhort to be sober minded.
 In all things shewing thyself a pattern of good works: in doctrine shewing uncorruptness, gravity, sincerity,
 Sound speech, that cannot be condemned; that he that is of the contrary part may be ashamed, having no evil thing to say of you.
6) Our doctrine should be uncorrupt, not based on opinion or feelings but on the sure foundation of the Word of God.
7) The Bible doctrines are to be held in the most serious fashion. Not shakable or negotiable but of certainty.
8) Our doctrine should be how we live. Not say one thing and do another.
1Tim.1: Knowing this, that the law is not made for a righteous man, but for the lawless and disobedient, for the ungodly and for sinners, for unholy and profane, for murderers of fathers and murderers of mothers, for manslayers,
 For whoremongers, for them that defile themselves with mankind, for menstealers, for liars, for perjured persons, and if there be any other thing that is contrary to sound doctrine;
9) Our doctrine instructs us on what is sin.
Rom.16: Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them.
10) Doctrine is the basis for discernment and separation.