Doctrine matters to modern Christians. While less and less Christians are picking the church they attend based on particulars of their statement of faith, most still do. Even those who don't pick churches this way still have specific views that are of major importance to them.
Doctrine mattered to the apostles, too, but I suspect most modern Christians would be surprised at which dotrines matter most.
The apostle Paul explains them to Titus in the following passage:
Not what you normally see in a "statement of faith," is it? Notice all the "to be's" rather than "to believe's"? (Well, you had to notice, since I highlighted them all; sorry.)
This is Paul's definition of "sound doctrine."
Other apostles agree:
These things line up with Titus 2:1-10, which makes sound doctrine to be the teaching that produces godly living.
Paul emphasizes this in the 2nd letter to Timothy:
The foundation uniting Christians is their choice to depart from iniquity!
This is important. For hundreds of years Christians have been fighting over doctrine, and their idea of sound teaching has to do with beliefs about eternal security, the nature of salvation, the Trinity, spiritual gifts, etc. I even heard about one denomination that divided over whether churches were allowed to support orphanages!
In the meantime, the way Christians live has become less and less important.
Ignatius of Antioch was appointed a bishop by the apostle John. The church he led was the apostle Paul's home church (Antioch). He gave his life for Christ—in great joy— in A.D. 110. On the way to his martyrdom, he wrote:
Ignatius obviously picked up Paul's emphasis on behavior. It's not a surprise, considering his position as the lead elder of Paul's home church.
However, I think we underestimate how much we've lost over the centuries. The sentence I just quoted for you isn't too shocking. You can read it and think, "Okay, the early church had a certain emphasis on behavior, and today we overemphasize theology."
However, the sentence before the one I just quoted is shocking.
We can temper this a bit. The Greek language Ignatius wrote in has both a "continual" and a "punctiliar" sense to its verbs. Ignatius is using the "continual" sense, the same way John did in 1 John 3:7. This passage could be translated, "No man making a profession of faith goes on sinning."
Even if we temper what Ignatius said, what about us today?
When a person joins one of our churches, it is normal for him to be shown a statement of faith and agree to it. But how many churches inform their members that a person who makes a true profession of faith stops sinning?
Worse, our statements of faith generally consist of doctrines that can be taught in a classroom but have little or no effect on our lives.
Look at Paul's "statement of faith" in Titus two. Its doctrines would make for a very short new person's class, but it asks a lot concerning our way of living.
Ignatius is not unique. Reading the writings of the earliest Christians can be a breathtaking experience for a modern Christian.
Justin Martyr, for example, writing in A.D. 150, said:
Doctrine needs to be practical. It is doctrine that is according to godliness (1 Tim. 6:3) that matters.
Modern Christianity has wandered from that, and our doctrine is very impractical. It's easily taught in a classroom, but it has no or little application to real life.
On the last day, Jesus will ask us about what we've done, not how well we've understood theology (Matt. 25:31-46).
We've grown too smart for our own good—or for doing good.
Paul knew what would happen to those who lost the focus on a transforming righteousness imparted from God. He has us pegged when he says:
It's time for us to refocus.
Every early church had a set of basic doctrines it required members to accept. It was called The Rule of Faith. It's like our modern statement of faith, but usually more basic.
And the early church never lost sight of the fact that behavior is more important than belief.
First, the doctrine that will be on many minds if you've read the page above is: What about salvation by faith?
There's also a page on the grace of God that is a logical extension of salvation by faith. Finally, the doctrines of faith, grace, and the judgment are combined together in one long page called Sola Fide.
There's a very practical teaching on the Word of God. It is clear, it is Scriptural, and it's historical, but it's flies in the face of some modern traditons enough that I've added an objection page and a discussion page.
Baptism is important to cover. It's a controversial topic today, but there's not any other doctrine so clear from both a historic and Scriptural standpoint.
I also cover some common objections to that doctrine.
As an aside, frequently asked about, we have a page on the early Christian understanding of the baptism of the dead as mentioned in 1 Corinthians 15:29.
Once that's resolved, The History of the Trinity is particularly worth going through! The Creed or Apostles Creed that most Christians consider the fount of orthodoxy addresses only the issue of Jesus Christ, his purpose, and his divinity. (I agree that the Nicene or Apostles Creed needs to be the basic theology that all Christians must agree with.)
I have now done a full page on TULIP, the five points of Calvinism, as well. There is also a page on the substitutionary atonement that was invented by St. Anselm and Thomas Aquinas during the Middle Ages.
I've been studying Nicea and the early church with particular reference to the Trinity for 19 years, and I believe those few pages are the most thorough and accurate treatment of the doctrine of the Trinity on the internet.
Another issue modern Christians struggle with and argue about concerns the role of the Law of Moses. It can be difficult to reconcile Jesus' statements in Matthew 5:17-20 with Paul's teachings on the Law, but it was no problem for the early churches.
Particularly interesting is their approach to sacrifices. You'll find much on that from the early Christians themselves in the quotes section.
A pet peeve of mine is the dishonest history being paraded around by Sabbath-keeping denominations. We can disagree on doctrine and get along, but when you make up history and deceive people, God will send you to hell. The story of how the Church began to honor the 1st day (or rather, 8th day) has been greatly distorted by false teachers. I give you the real and verifiable Sabbath to Sunday story.
There's some Roman Catholic doctrines that I want to address here because of their interest to a wide group of people. Infant Baptism is a major one, and it's history is thought-provoking and challenging. Purgatory is another that has some interesting history—not about Purgatory itself—to teach us.