My issue is that non-denomination have really very few core beliefs. As I said, everyone can have a different idea on what is right/wrong on what you call "peripheral" doctrines which can lead to false doctrine and heresy.
This is false. The nondenominational churches that I've had experience with were much more solid in their biblical foundations than most of the denominational churches I've been in. I think that the old institutions have done quite well in not establishing their members in the gospel and in promoting false doctrines and heresies.
Jesus said the Holy Spirit will teach us:
John 14:26 But the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things and bring to your remembrance all that I have said to you.
And John writes:
1 John 2:26-27 26 I write these things to you about those who are trying to deceive you. 27 But the anointing that you received from him abides in you, and you have no need that anyone should teach you. But as his anointing teaches you about everything, and is true, and is no liejust as it has taught you, abide in him.
We don't need governing bodies between God and men telling us what God's will is. I don't advocate anarchy, and accountability to God and to one another is very important, but this idea that we need these massive institutions and their teaching, that have nothing to do with abiding in fellowship with the Father and with Jesus Christ, is ridiculous.
There are certain things that I certainly would not consider to be a "peripheral" that non-denomination churches do. There is also the issue of the pulpit preaching of things that are are merely one type of interpretation (such as dispensationalism).
This goes both ways. Just because you don't think that dispensations are valid, and I think I can make a solid case from scripture that they are, doesn't make it a heresy or make it a problem if it is preached from a pulpit.
While I think that we are missing out on important revelations in scripture if we deny the dispensations, believing dispensations, or not, has nothing to do with salvation or living a life directed by the Holy Spirit and pleasing to God. In other words, it is "peripheral".
When people in a church are not unified in their theology I can only see it as a nightmare. What do the children, learn, exactly?
Why do we require massive, expensive, and, many times, corrupt institutions in order for people to be unified in their theology? You may feel you need it, and I won't condemn you for it because people's personalities and preferences are different. If that suits you, ok, but don't think that the rest of us are somehow lost in a chaotic nightmare of uncertainty, because we don't.
As far as what the children learn, they learn the gospel, they learn the Christian and the Jewish scriptures. They learn to love others the way that Jesus calls us to, and they learn to have the heart of a servant for other people.
They don't learn new age gibberish or paganism paraded as Godliness and they sure don't learn to ask dead people to intercede for them when the scripture tells us:
1 Timothy 2:5-6 5 For there is one God, and there is one mediator between God and men, the man Christ Jesus, 6 who gave himself as a ransom for all, which is the testimony given at the proper time.
I'm speaking generally here. I know there are some kooky, heretical churches, but you get that in denominations, too. A person has to be discerning and walk with Jesus so that they don't get led away by false teaching in any church.
John tells us in 1 John that he was writing the epistle so that we will have full joy, fellowship with other believers and with the Father and the Son, so that we won't sin, so that we have assurance that we have eternal life in Jesus,
so that we will test the spirits and recognize those who seek to lead us astray, and so that we will have confidence when He returns and not be ashamed.
It's each of our responsibilities to search the scriptures and make sure that what we are being taught matches what God has revealed to us, just like the Bareans did:
Acts 17:10-11 10 The brothers immediately sent Paul and Silas away by night to Berea, and when they arrived they went into the Jewish synagogue. 11 Now these Jews were more noble than those in Thessalonica; they received the word with all eagerness, examining the Scriptures daily to see if these things were so.
Your verses are about Christians as a whole (believers in Christ). The early church became the Roman Catholic/Orthodox church.
Yes, the verses are about the whole church, i.e., the body of Christ, and no, the body of Christ did not become the Roman Catholic/Orthodox Church. There were fellowships and communities of Christians in many places before, during, and after Rome decided to co-opt "Christianity" for their own purposes. While I believe that there are many Christians within the RCC, I don't see anything about the institution itself that would lead me to conclude that it is truly led by Jesus.
^ "Man-made" theology comes from interpretation of the Bible. Some interpretation can be Spirit-led. Some can be by ones wants - some who use snippets of scripture rather than context to build up their validity.
Why was the Nicene Creed developed in the first place?
Some of it comes from incorrectly interpreting scripture, true. Much of it has absolutely nothing to do with scripture at all. Praying to something other than God is one, elevating people to the status of saints is another (the scriptures tell us that all that are redeemed by Jesus are saints). Things like forbidding dancing, makeup, or women in pants are some others. Then there is denying the gifts of the Spirit, and no, I am not claiming that everyone is to speak in tongues or that we are to be carried away in the flesh by our emotions, but to deny the work of the Spirit seems to match this scripture:
2 Timothy 3:1-9 1 But understand this, that in the last days there will come times of difficulty. 2 For people will be lovers of self, lovers of money, proud, arrogant, abusive, disobedient to their parents, ungrateful, unholy, 3 heartless, unappeasable, slanderous, without self-control, brutal, not loving good, 4 treacherous, reckless, swollen with conceit, lovers of pleasure rather than lovers of God, 5 having the appearance of godliness, but denying its power. Avoid such people. 6 For among them are those who creep into households and capture weak women, burdened with sins and led astray by various passions, 7 always learning and never able to arrive at a knowledge of the truth. 8 Just as Jannes and Jambres opposed Moses, so these men also oppose the truth, men corrupted in mind and disqualified regarding the faith. 9 But they will not get very far, for their folly will be plain to all, as was that of those two men.
The Nicene Creed was to counter the doctrine of Arianism which taught that Jesus was not divine, but merely a created being.