What are the best ways to refute the position that Paul started Christianity?

The article you cite at http://www.sullivan-county.com/news/paul/paul.htm is a poorly constructed attempt to prove a point. To counter this you need to know its weaknesses, but also its strengths.

First of all, avoid misquoting the article, as in "Paul started Christianity" - the article asserts that Paul started Christianity as we know it today, an entirely different matter. It is crystal clear that Christianity existed in Jerusalem and perhaps even Rome before Paul. Knowing exactly what is being asserted enables you to know what to assert in return.

As for Acts of the Apostles, it is the consensus of scholars that Luke the physician was not really the author. As with all four New Testament gospels, Acts was originally anonymous. It (and Luke's Gospel) was attributed to Luke later in the second century on the twin assumptions that it ought to have been written by someone who knew Paul quite well, also by a gentile, and finally by a moderately well educated person who could write in high Koine. Luke was the one person whom the Church Fathers felt fitted these criteria. In fact, Acts is now believed to have been written around the end of the first century or early in the second century. Authorship by a companion of Paul is also inconsistent with the historical and theological differences and discrepancies from Paul's letters. So, Acts of the Apostles should be set aside when determining the influence of Paul on later Christian belief. You are also right in that 2 Peter must be put aside as a second-century forgery.

Strange as it may seem, we know little about Paul's theology - his view of Christianity. He does say that the risen Jesus was seen by Cephas (Peter), then the twelve, then by more than 500, most of whom were still alive, then by James and all the apostles, and finally by himself. Christianity is based on very different accounts found in the New Testament gospels, including the obvious difference that there were at that time only eleven disciples.

Scholars do detect elements of Gnosticism in certain passages in Paul's epistles, and his statement that he never learnt the gospel from humans but by revelation is consistent with a gnostic approach. Modern Christianity is not gnostic, so whatever influence Paul had, it was not to influence Christianity towards Gnosticism.

Having said that Luke/Acts should be put aside as pseudepigraphical when considering what Paul taught, it is at the same time true that Christianity today reflects much of what we find in Luke's Gospel and Acts of the Apostles. With the possible exception of the author of Mark's Gospel, no other evangelist has has more influence on the direction of the Christian faith.

The reasoning behind the saying that Paul began Christianity is in:

Acts 11:22 Then news of these things came to the ears of the church in Jerusalem, and they sent out Barnabas to go as far as Antioch. 23 When he came and had seen the grace of God, he was glad, and encouraged them all that with purpose of heart they should continue with the Lord. 24 For he was a good man, full of the Holy Spirit and of faith. And a great many people were added to the Lord. 25 Then Barnabas departed for Tarsus to seek Saul. 26 And when he had found him, he brought him to Antioch. So it was that for a whole year they assembled with the church and taught a great many people. And the disciples were first called Christians in Antioch.

Up until this time Jesus followers were known as 'people of the way.'

Acts 19:9 But when some were hardened and did not believe, but spoke evil of the Way before the multitude, he departed from them and withdrew the disciples, reasoning daily in the school of Tyrannus.

and:

Acts 19:23 And about that time there arose a great commotion about the Way.

What are the best ways to refute the position that Paul started Christianity?

If you read the book of Acts, you get the impression that the beginning talks about the Apostles and what is happening in Jerusalem and ends with the focus on Paul and what is happening outside of Jerusalem. Many people see in this a hijack of Christianity, a diversion of Christianity, and even a repackaging of Christianity to a new market by Paul.

Jesus told his disciples that he was giving them the new covenant.

Mark 14:24 and he said to them, `This is my blood of the new covenant, which for many is being poured out;

Jesus told his disciples that they would be bringing the good news (gospel) of the Kingdom to the whole world.

Mark 16:15-18 And he said unto them, Go ye into all the world, and preach the gospel to every creature. He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.

The original plan was for Israel to receive her King and spread the gospel over the whole earth. However unlike the first covenant when the entire nation of Israel accepted the covenant, this covenant was not received by the nation.

Paul writes that one of the reasons salvation had come to the gentiles was to provoke the Jews to jealousy.

Romans 11:11 I say then, Have they stumbled that they should fall? God forbid: but rather through their fall salvation is come unto the Gentiles, for to provoke them to jealousy.

One can also see that Paul was not instructed like the other Apostles. He himself calls himself the Apostle to the gentiles.

1 Timothy 2:7 Whereunto I am ordained a preacher, and an apostle, (I speak the truth in Christ, and lie not;) a teacher of the Gentiles in faith and verity.

Paul did perform an occasional baptism (cited in the commission of Jesus to his disciples), but it was not part of what he was called to do.

1 Corinthians 1:17 For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.

Paul had every hope and expectation that his ministry would be short lived and that the Kingdom would come to Israel. We can see his expectation in his use of the word "we".

1 Thessalonians 4:17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.

I think we can take from the words of Jesus in what is called the great commission that his kingdom was always intended to be worldwide.

Paul was called to go to the gentiles. The failure of Israel to receive the kingdom for the last 2,000 years makes the ministry of Paul seem larger. As a result, many make claims about Paul's work and motivation that are difficult to substantiate.

The book of Revelation describes a future where a faithful remnant of Israel will receive her King and the prophecies for Israel will be fulfilled.