Critics make this claim to try and support their assertion that Paul somehow hijacked Christianity away from the Mosaic Law and into his own "grace theology." But the answer about who wrote most of the NT is easy enough to come to. All we have to do is the math.
The information used below comes from the oldest sources we have: the Greek manuscripts of the New Testament. As you may know, the authors of the NT wrote in Koine Greek, which was the lingua franca of their day. The word numbers may vary slightly from source to source, but these are not going to change our final results.
So who are our Top Three writers?
The New Testament has a total of 138,020 Greek words in the Analytical Greek New Testament (AGNT).
Of those 138,020 words, the most prolific writers of the New Testament were:
Author Books # of Words % of NT
Luke Gospel of Luke 37,933 27%
Paul Romans 32,407 23%
Other NT facts:
Gospel of Matthew 18,345 words
Gospel of Mark 11,304 words
Gospel of Luke 19,482 words
Gospel of John 15,635 words
For a total of 64,766 words
The four Gospels which tell us about the life and words of Jesus comprise almost 47% of the New Testament.
The next largest book after that is Luke's Book of the Acts of the Apostles, with 18,451 words, or 13% of the New Testament.
After Acts comes the Book of Revelation, which contains what many Christians consider yet-to-be-fulfilled prophecy in the New Testament. Revelation has 9,852 words, or about 7% of the NT.
The longest epistle is Paul's letter to the Romans, with 7,111 words, while the shortest is John's epistle of 3 John, with 219.
The only website I found with a list of the number of words in the Greek New Testament was this one. The data is based on the Analytical Greek New Testament (AGNT):