The first three gospels are called the "synoptics", from a Greek phrase meaning "seen together", because they put the events of Jesus' life in the same order and have many of the same stories and sayings, often in the same or very similar words.
The usual way of explaining this is that Mark was written first, and that the authors of Matthew and Luke, acting independently, used Mark plus a collection of sayings called the Q document and additional material unique to each called the M source (Matthew) and the L source (Luke).
This isn’t a conclusive argument, because early Christians like Paul often relied on oral tradition rather than direct quotation from the New Testament, but the fact Paul’s epistles from the 50s never clearly refer to the Gospels is at least suggestive. I should note that there are some possible exceptions to the above. 53, but this passage probably is not a quotation from Luke’s Gospel. Luke was a travelling companion of Paul (Acts -17, 20:5-15, 21:1-8, 27:1-; cf. “The Brother Whose Praise Is in the Gospel” Second, Paul makes a mysterious reference in 2 Corinthians to a “brother whose praise is in the gospel” (literal translation). However, the passage is ambiguous, and we can’t be confident of this.
First, in 1 Corinthians -25, Paul quotes Jesus’ words of institution for the Eucharist, and the form of words he uses is the one found in Luke -20, not the one found in Matthew -28 or Mark -24. In fact, the passage is normally taken as a reference to a brother Christian who was famous for the gospel—not for having written a Gospel (some Bible versions even translate the verse that way).
“The Worker Is Worth His Wages” Third, 1 Timothy states: [T]he scripture says, “You shall not muzzle an ox when it is treading out the grain,” and, “The laborer deserves his wages.” The first quotation is found in Deuteronomy 25:4 and the second is found in Luke 10:7. This would suggest that the Gospel of Luke was in circulation in the A. 60s, but Ehrman’s point is still fair that Paul’s letters from the 50s don’t contain any clear references to the Gospels.
Dating the Bible has been debated and these four tables give the most commonly accepted dates or ranges of dates for the Old Testament/Hebrew Bible, the Deuterocanonical books (included in Roman Catholic and Eastern Orthodox bibles, but not in the Hebrew and Protestant bibles) and the New Testament, including—where possible—hypotheses about their formation-history. Table II treats the Old Testament/Hebrew Bible books, grouped according to the divisions of the Hebrew Bible with occasional reference to scholarly divisions. Table IV gives the books of the New Testament, including the earliest preserved fragments for each.
This table summarises the chronology of the main tables and serves as a guide to the historical periods mentioned.