Why are the New Testament quotes of the Old Testament different?

Answer:

"The voice of one crying in the wilderness: 'Prepare the way of the LORD; make straight in the desert a highway for our God'" (Isaiah 40:3).

"For this is he who was spoken of by the prophet Isaiah, saying: "The voice of one crying in the wilderness: 'Prepare the way of the LORD; make His paths straight'"" (Matthew 3:3).

"The voice of one crying in the wilderness: 'Prepare the way of the LORD; make His paths straight.'" (Mark 1:3).

"As it is written in the book of the words of Isaiah the prophet, saying: "The voice of one crying in the wilderness: 'Prepare the way of the LORD; make His paths straight.'" (Luke 3:4).

"He said: "I am 'The voice of one crying in the wilderness: "Make straight the way of the LORD,"' as the prophet Isaiah said" (John 1:27).

If you accept that the New Testament was inspired by the same Holy Spirit who had the Old Testament written, your question would have been worded quite differently. Peter said, "Knowing this first, that no prophecy of Scripture is of any private interpretation, for prophecy never came by the will of man, but holy men of God spoke as they were moved by the Holy Spirit" (I Peter 1:20-21). By saying that the New Testament writers got something wrong, you are either charging the Holy Spirit with making a mistake or charging the apostles with being uninspired.

The charge is without foundation. The Old Testament is written in Hebrew and that language contains no punctuation. The New Testament is written in Koine Greek and that language contains no punctuation. The punctuation was added by the interpreters as they translated the Hebrew and the Greek into the English and other modern languages. The punctuation helps clarify the meaning for the readers of English.

Now the quotes of the Old Testament found in the New Testament come to us through two steps. The Old Testament passage is in Hebrew. It is then translated into Greek under the inspiration of the Holy Spirit. Finally, the quote is translated into English.

Why are the punctuation and wording sometimes different between the Old and New Testament? First, most translations are done by groups of people. The people working on the Old Testament are Hebrew scholars and the people working on the New Testament are Greek scholars. Different people will translate the same source language differently. The meaning will be basically the same, but there will be slight variations. This is because words in every language contain multiple shades of meaning; yet, it is rare to find a word or phrase in one language that precisely matches the word or phrase in another language in all its shades of meaning. Hence translation always involves compromises, and different translators will make different choices.

Second, chapter and verse numbering did not come along until long after the Bible was written. A quote from the Old Testament is perfectly correct even though the entire verse is not quoted. Notice that Matthew, Mark, and Luke all make the same quote, but all only use the first part of Isaiah 40:3.

John's account is a record of what John the Baptist said in reference to Isaiah. In this particular case, we need to understand that an allusion to a quote is still essentially a quote. The difference is that Matthew, Mark, and Luke are applying the quote to a third person (John the Baptist) just as Isaiah 40:3 does. However, John tells us that John the Baptist referred to this verse in application to himself. That change from the third person to the first person requires a slight change in wording in order to remain grammatically correct in Greek. The meaning of the quote, however, remains the same.