Know doctrine, know life. No doctrine, no life.
Ideas have consequences.
What we think affects how we live. Orthodoxy (right belief) leads to orthopraxy (right behaviour).
Sometimes this is obvious and sometimes it isn’t. We sometimes struggle to apply the weightier themes in the bible to specific aspects of our daily lives. Does it really matter what I think about doctrines like election, the trinity or substitutionary atonement? Does a correct understanding of these things really affect how I live? Can’t I just love Jesus and be good?
Lets see what the bible has to say.
In Titus 3:4-8, Paul sets forth some of the big, old, orthodox Christian doctrines - the nature of God, salvation, grace, mercy, regeneration, indwelling of the Spirit, justification, adoption, eternity. We are then told to insist on these things. Why? Notice the “so that” in verse 8. So that we will devote ourselves to good works.
 But when the goodness and loving kindness of God our Savior appeared,  he saved us, not because of works done by us in righteousness, but according to his own mercy, by the washing of regeneration and renewal of the Holy Spirit,  whom he poured out on us richly through Jesus Christ our Savior,  so that being justified by his grace we might become heirs according to the hope of eternal life.  The saying is trustworthy, and I want you to insist on these things, so that those who have believed in God may be careful to devote themselves to good works. These things are excellent and profitable for people.
(Titus 3:4-8 ESV)
Here is what insist means according to some online dictionaries:
Demand something forcefully, not accepting refusal
To assert or demand (something) vehemently and persistently
To dwell with earnestness or emphasis
To be emphatic, firm, or resolute about something intended, demanded, or required
These doctrines are not abstract ideas we should give up in favour of simplistic notions of love and unity with those who disagree, or because they are difficult and seem irrelevant. These are the very ideas that help us grow more like Christ. We are to hold on to these firmly.
Similarly, in Philippians 1:9-10 it says that when we approve (or understand) what is excellent (right doctrine), we live pure and blameless lives.
 And it is my prayer that your love may abound more and more, with knowledge and all discernment,  so that you may approve what is excellent, and so be pure and blameless for the day of Christ,
(Philippians 1:9-10 ESV)
The New Living Translation makes this even clearer. Again, note the “so that”
 I pray that your love will overflow more and more, and that you will keep on growing in knowledge and understanding.  For I want you to understand what really matters, so that you may live pure and blameless lives until the day of Christ’s return.
(Phil 1:9-10 NLT)
(Phil 1:9-10 NLT)
2 Peter 1:2-3
In these verses we see that God’s power works through knowledge of him to grant grace, peace and all we need for life and godliness.
 May grace and peace be multiplied to you in the knowledge of God and of Jesus our Lord.  His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence,
(2 Peter 1:2-3 ESV)
1 Corinthians 6:1-3, 15; 7:15
In these verses Paul is addressing some of the sinful practices in the Corinthian churches. In doing so, he repeatedly asks “Do you not know...?” and refers to a particular doctrine, making it clear that a right understanding of these seemingly obscure doctrines should affect how we now live.
[6:1] When one of you has a grievance against another, does he dare go to law before the unrighteous instead of the saints?  Or do you not know that the saints will judge the world? And if the world is to be judged by you, are you incompetent to try trivial cases?  Do you not know that we are to judge angels? How much more, then, matters pertaining to this life!
(1 Corinthians 6:1-3 ESV)
 Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality,
(1 Corinthians 6:9 ESV)
 Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make them members of a prostitute? Never!  Or do you not know that he who is joined to a prostitute becomes one body with her? For, as it is written, “The two will become one flesh.”
(1 Corinthians 6:15-16 ESV)
 Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.  Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God? You are not your own,  for you were bought with a price. So glorify God in your body.
(1 Corinthians 6:18-20 ESV)
2 Corinthians 8-9
In the middle of each these chapters, where Paul is addressing how the Corinthians should financially support the church in Jerusalem, he stops to apply some theology which should generate a response in them by their generosity. Reading these two chapters, It is striking how these verses are injected into the middle of all the practical stuff.
 For you know the grace of our Lord Jesus Christ, that though he was rich, yet for your sake he became poor, so that you by his poverty might become rich.
(2 Corinthians 8:9 ESV)
 As it is written,
“He has distributed freely, he has given to the poor;
his righteousness endures forever.”
(2 Corinthians 9:9 ESV)
Read and study your Bible. Think hard. Read books. Listen to sermons. Go to bible studies and conferences. Learn church history and the creeds.
Knowing this stuff makes a difference.