14 I was advancing in Judaism beyond many of my contemporaries and was extremely zealous for the traditions of my fathers. -Galatians 1:14 BSB
The phrase “advancing in Judaism” is most likely in reference to being more rigid and strict in keeping the Law of Moses and the traditions of men, which will eventually end up canceling the Scriptures.
Paul’s zeal pushed him to the extreme, causing him to hunt down Christians wherever they went.
Having zeal is good but must be accompanied by knowledge of the truth. Paul, also known as Saul, had that zeal, but he was misguided since he relied on man-made tradition. The traditions handed down to the Jews trumped the Scriptures, which pushed him to persecute the saints at the time, see Acts 22:1-5.
Jesus condemns man-made tradition
Jesus replied, “And why do you break the command of God for the sake of your tradition? -Matthew 15:3 BSB
Zeal without knowledge is not good
2 For I testify about them that they are zealous for God, but not on the basis of knowledge. 3 Because they were ignorant of God’s righteousness and sought to establish their own, they did not submit to God’s righteousness. -Romans 10:2-3 BSB
Adding to God’s Word will delude one’s understanding of God’s standard and from walking in the truth. If one adds extra-biblical sources to the Word of God, it will be favored over the Scriptures since dark spiritual forces are behind it.