I’m somewhat doing these in order … so this one is definitely number 2.
An idolatrous version of Jesus (one made in our own image or likeness instead of the true God) is and should be the most central problem facing Christianity. This is the first determining factor whether or not you can be saved and in fellowship in the Church of God.
Throughout the New Testament, it is clear that the conversion to following Christ can be summed up in one word: repentance. To repent means to turn, like a 180. This is a turning from self and, by extension, sin; and turning to Christ and, by extension, righteousness. This is primarily an issue of the heart. To follow Christ I must, by necessity, cease trusting and clinging to my own self and my rights and thoughts and motivations, and instead begin placing all my trust and hope into the power of the resurrected Christ.
But the moral implications were never ignored in scripture. They were expressly taught. Paul says in Acts 26 that his gospel to Jews and Gentiles was to “repent, turn to God, and do works befitting repentance.” In fact, the whole New Testament expresses a conversion experience that demands the immediate change of the entire person. Not that there isn’t maturity available in Christ as we go on, there absolutely is and we must continue to move on and upwards, but even the initial change seen and expected in the Christians of the Bible would seem to many of us as saintly, at the very least.
In other words, a complete and immediate change to living righteously upon conversion was considered NORMAL Christianity by the writers of the New Testament.
So you can see how the belief that Christ gives grace to enable us to sin without consequence becomes more than problematic to the early Christians, even abominable, to use a harsh but appropriate word.
It doesn’t seem so big a deal to us today because we do not have the same extreme expectation, at least in the West, of a complete immediate change, first of the heart, then of the entire person. But to the first century apostles it was the very opposite of the gospel they preached.
To go to some verses to help us see how they viewed this, we could start with Romans. In the midst of Romans 3, as Paul was explaining his doctrine on the Jews and the common state that we have all sinned, he makes the following statement: “And why not say, ‘Let us do evil that good may come’?—as we are slanderously reported and as some affirm that we say. Their condemnation is just.” Paul considered it slander to even suggest and affirm that he would teach such a thing and then states that their condemnation was completely justified.
To even stay in Romans, Paul deals with it again in chapter 6, in the midst of his doctrine on grace, stating “Shall we continue in sin that grace may abound? Certainly not!” at the beginning of the chapter and then states it AGAIN in v.15 “Shall we sin because we are not under law but under grace? Certainly not!” Realize two things here. For Paul to repeat himself this way, twice in writing, was to further underscore the point, to make it as strong as he could make it. And secondly, the Greek language is even further expressive and harsh than the English language used to interpret it.
(Another thing to note in chapter 6 is that we had died to sin (i.e. set free from it) at baptism, which is at conversion and commitment, being “baptized into His death”, and that Paul describes the process of sinning now as a Christian as “presenting your members” to unrighteousness, which is not being bound to sin any longer but choosing to offer yourself as an instrument of sin even though you are spiritually free from it. The answer overcoming sin is a little earlier in chapter 6, that you “reckon yourself” dead to sin. The problem is not the state of being free, therefore, to overcome sin, but the renewing of our mind to actually believe and fully trust that something so amazing beyond our ability or merit had been done even at conversion, to be completely set free from the power of sin.)
Well, let’s get out of Romans and into a more scary letter, Jude. The whole letter of Jude was written to combat this problematic doctrine. Jude is writing to them because he is concerned for their salvation, that they should contend for the faith “delivered to all the saints” because “certain men have crept in unnoticed, who long ago were marked out for this condemnation, ungodly men, who turn the grace of our God into lewdness and deny the only Lord God and our Lord Jesus Christ.”
Even in this introduction, we see the connection between the doctrine and the very denial of Christ.
Jude goes on in his letter to give examples from the Old Testament and the book of Enoch to show God’s hate and subsequent wrath for sin. Most people don’t normally read this letter, but it is just as true as “salvation by faith and grace alone.”
Jude also considers them “marked out for condemnation” who believe and teach this doctrine, which again supports the absolute intolerance for this doctrine, biblically, and why it even becomes an issue of fellowship.
I do love, as a slight tangent, the end of the letter, where Jude describes how to deal with such people. “And on some have compassion, making a distinction; but others save with fear, pulling them out of the fire, hating even the garment defiled by the flesh.” In other words, there is a place for both compassion and fear and hate in dealing with these things. It is wisdom to remember the validity of them all.
One more scripture to underscore that this was something that became grounds for breaking off fellowship. 1 Corinthians 5 details a situation in which the local assembly had continued to accept a man who had done a pretty reprehensible thing, and they were proud of it. More implied than stated, this is the same doctrine that grace can accept such a man steeped in such sin.
Paul’s instruction to them was to cease all fellowship with such an individual, literally calling it “handing him over to Satan.” Then Paul goes on to remind them that this is a general doctrine for the Church. “I wrote to you in my epistle not to keep company with sexually immoral people. Yet I certainly did not mean with the sexually immoral people of this world, or with the covetous, or extortioners, or idolaters, since then you would need to go out of the world. But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner—not even to eat with such a person. For what have I to do with judging those also who are outside? Do you not judge those who are inside? But those who are outside God judges. Therefore ‘put away from yourselves the evil person.’”
Paul’s concern was not that you break off all fellowship with the sinners of the world. Sinners will be sinners, and we should not expect something different from them. Paul’s strong concern was that someone who says they believe in Christ would participate in sexual immorality, covetousness, idolatry, reviling, drunkenness, or extortion, should be put out of the local assembly, the equivalent of not even eating with such a person.
A redemptive aspect of this which may help some of you feel better is that this specific person mentioned in chapter 5 was allowed back into the church after he repented and he had “suffered enough” (Paul speaks of him again in 2 Corinthians). Additionally, most of 1 Corinthians is a rebuke against selfish and divisive behavior, and yet Paul’s expectation was that they would repent, to turn again to what he had taught them in righteousness and love, and all evidence suggests they did.
You can stop reading now, if you’d like. I know it is getting TLDR, but there was one example on my heart to share that may shed some light on this.
This doctrine, that the grace of God allows for sin or even justifies it, is willful denial of the lordship of Christ, His authority and the work done on the cross and in His resurrection.
Let’s say my son, Micah, disobeys but immediately recognizes that he was wrong and accepts my punishment as just. (Believe it or not, it does happen 
). In that case, although disobedience occurred, relationship never really suffered, and while punishment does happen, it is quick and rather painless and things move on quickly again to intimacy because the correction to truth was almost instantaneous. But let’s say that when I tell Micah to do something, he stands there, crosses his arms, looks me in the eye and says, defiantly, “No.” (Rare, but that happens, too) In that case, Micah is denying my right to give him instruction and commands, denying his obligation and responsibility to obey, in practice and in attitude denying that I am his father at all, asserting his independence from my authority over him. All of this is a lie and a denial of truth. As a good parent, this is beyond problematic, it is dangerous to his very spiritual health, and all of my energy and attention is now on this problem. My response must be decisive and intentional, and the punishment for such a thing severe.
Because I love him.
If I didn’t love my son, then such a defiance would be tolerable, even funny. But because I love him so much, I want the best for him, to grow to be a man of character and integrity, I must by nature treat such defiance with severity.
And once Micah’s attitude has changed, the correction has occurred. The punishment itself, or even the severity, is not the goal, but the correction in attitude. Once such a correction occurs, our relationship is fully restored to truth, and in intimacy I assert with words and instruction the deep love that obligated my severity.
God is the same. Paul’s goal in 1 Corinthians chapter 5 was not the punishment, but the correction, the repentance, and it happened. Paul’s rejection of fellowship for this believer, even all believers who participate in such things believing it is fine, is an act of love. And God’s chastening of us is as sons and daughters. If he didn’t seek the best for us, our righteous character and integrity, He wouldn’t be God and we wouldn’t be his children. We would be bastards.
As C.S. Lewis says, to want a God who lets us do what we want and excuses our sin is to want a God who loves us less, not more.
Peace.