"For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery" (Galatians 5:1, ESV)
Fear is the irrational rationalism of those who are bound by sin. To put it another way, fear is the fruit of seeing God as only wrath without the possibility of Him being otherwise. What is odd about this kind of fear is that it does not feel like fear to those who have it. It feels like freedom. Let me explain.
Sin is powerful. It is not simply powerful from a physical standpoint, dominating lives and actions of many, but sin is also psychological. It blinds men from seeing true freedom in Christ and distorts their thinking about what freedom actually is. Sin distorted freedom and made men understand freedom as being free from the moral restraints of God. Therefore, every rejection of the gospel is a distorted understanding of freedom. Men feel that they are free to reject God and the gospel as some sort of act of free will. What they do not understand, however, is that their rejection of Christ (their expression of free will) is actually the proof and fruit of their being bound (not free) by and to sin.
What we see in the rejection of the gospel is fear. But not fear of God (though this is true) but in principle, we see the fear of freedom. I know that sounds odd that some would fear freedom but Paul's teaching of the gospel of grace teaches us this very fact. That is the oddity and insanity of sin: it keeps men bound to the notions of freedom that actually destroy them (eternal separation from the Lord) and moves them to constantly reject the freedom that is their only hope, Christ alone. For Paul and indeed the whole Bible, no person is a true human being until they are reconciled to the Lord who created them. Therefore, the old adage, "to err is human"in not true! It should be rightly corrected in this way - to err is to be rebellious humanity." Humanity became sinful by the actions of Adam and Eve, but original humanity was perfect. Thus, "to be perfect is to be human."
Yet, the story of the Bible is that man, alone, could not achieve such perfection that the Lord required. The gospel of grace (the message of the life, death, resurrection and ascension of Christ) was preached to restore humanity's relationship with God thereby restoring in us the essence of true humanity - the ability to choose God above and beyond sin. This is what it means to, not only be truly human, but this is what it means to be free! It means being reconciled back to the Lord by faith in Jesus Christ such that we can choose the Lord over self, sin and Satan. The gospel frees us from the bondage of our distorted humanity and its false view of freedom because of sin, so that we can live out our reconciled humanity in the freedom of obedience.
Now, in Christ, there is no longer the fear that crippled us to reject Christ based on the false notion of freedom (doing what we want to do). We have been freed from that distorted view of freedom so that we can, in Christ, live in the freedom that God intends. This is what Paul means in Galatians 5:1 (the verse cited at the top). In other words, we can now live for the Lord without the fear of rejection due to the presence of sin in our lives since we saved by grace and, by this grace, enabled, by the Holy Spirit, to live above the influence of sin. Paul shows us what this freedom is and what it looks like in Galatians 5:16, "But I say, walk by the Spirit, and you will not gratify the desires of the flesh."
There remains no need for fear for those who are in Christ Jesus, not simply because there is no condemnation (Romans 8:1) but also because there is now power to live for Him out of the perfection He restored in us in Christ. We no longer fear not being perfect because we have been perfected in Christ by faith. This truth not only empowers us to live for the Lord, but it fuels our passions to live for Him (Romans 6:1, 2). This is why we strive to please Him with every moment because He has freed us from wrath and this freedom produces a worship that is fleshed out in obedience (See Colossians 1:9-13)
We were freed, not simply to go to heaven. We were freed to choose Jesus over this world, every day and every moment of our lives. When we fail, however, we do not fear because the imperfections of our lives are not stronger than the perfection of His life in us.