Q. In studying, there is a major difference between the KJV/NKJV and the NASB on Acts 18.21. Why is there a difference?

A. The difference you refer to is where the KJV/NKJV contains the phrase “I must by all means keep this feast that comes in Jerusalem” where the NASB leaves it out. This is due, in many cases, to the fact that different versions use different manuscripts or if a majority of manuscripts carry the verse or not and so on. This is called “Textual Criticism” and the KJV bases its translation on different reasons than the NASB. However, even though the NASB leaves that phrase out based on their texts, it still does not mean that in other places it won’t tell you where and why he was going. In v22 it says “he went up” to greet the church and “to go up” is a phrase talking about going “up” to Jerusalem. In Acts 20.16 the NASB explicitly says that Paul was hurrying to be in Jerusalem if possible on the day of Pentecost. So, 20.16 basically says the same thing as 18.21. In Acts 24.17 it says that Paul went to Jerusalem at the time of that festival because he was giving alms and to present “offerings” in the Temple. Why would he do that? In Acts 18.18 it says that Paul “cut his hair” because he was keeping a vow, which was a Nazarite vow found in Num 6.1-21 and the offerings he gave are listed there. Acts 21.15-26 tells you he gave these offerings. Now, Pentecost was one of three mandatory festivals that the males were to attend every year (Exo 23.14-17). So, in my opinion, here is what happened. Paul cut his hair ending his Nazarite vow in Acts 18.18 It is the only vow in the Bible that ends when one cuts his hair. It then says in 18.21 in the KJV/NKJV  that he must go to Jerusalem. This was for 2 reasons, he was required to attend if possible (Exo 23.14-17) and he was ending his Nazarite vow and had offerings to give and he had to take his hair to the Temple to be burned (Num 6.1-21, Acts 21.26, 24.17). The scriptures are very clear as to why he went no matter what version you read, but in 18.21 the NASB leaves out the phrase found in the KJV/NKJV probably due to manuscript evidence, but other verses tell you why anyway so the basic truth of God’s word still is conveyed. By the way, I use the NASB myself but it is always good to read other versions as well to get the full flavor of what is being said. There is another thing to keep in mind, the original manuscripts written by the authors themselves were inspired by God, but copies upon copies were made from them  and human error will inevitably happen.  But God’s truth is still there and the concepts are still contained in the Bible. Some Bibles are strong in some areas, and weaker in another so it is always good to look at various versions and to get a working knowledge of the Hebrew and Greek. Also, look up words in concordances and lexicons and  have a basic idea of textual criticism as well. But, remember that the Holy Spirit will lead us to the truth regardless of education and  will help us use the tools mentioned above. We are all students of His word and on different levels so also keep this in mind when sharing.