A second common claim is that, if he had reliable knowledge of Jesus, he would have quoted him.
The conclusion that is drawn from these premises is that St. Paul was not a reliable source on Jesus.
Since St. Paul’s letters are among the earliest works of the New Testament, some proceed from there to argue either that historical knowledge of Jesus is impossible or even that he didn’t exist.
Such arguments are highly problematic.
The Second Premise
First, let’s consider the premise that Paul should have quoted Jesus if he had reliable knowledge of him.
Is that true?
It would be true if, in his letters, Paul was offering detailed catechesis on the life and ministry of Jesus (the way the Gospels do).
However, if Paul is not intending to offer detailed catechesis about the life and ministry of Jesus, he would have much less occasion to quote him.
The fact is that St. Paul’s epistles do not attempt to offer detailed catechesis. He is writing largely in a pastoral vein, dealing, for example, with various problems that have arisen in the churches he has founded or is planning to visit.
As a result, he would have much less occasion to quote Jesus. The only time it would be relevant for him to do so is if Jesus said something directly relevant to the problem he is dealing with.
Even then, he need not do so. Just because Jesus said something relevant does not mean that it must be quoted.
Christians today write on all kinds of subjects without being forced to quote everything Jesus said that might be relevant to the issue at hand.
The First Premise
Then there’s the first premise–that Paul never quoted Jesus.
Um, dude? 1 Corinthians 11?
 For I received from the Lord what I also delivered to you, that the Lord Jesus on the night when he was betrayed took bread
 and when he had given thanks, he broke it, and said, “This is my body which is for you. Do this in remembrance of me.”
 In the same way also the cup, after supper, saying, “This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.”
Is that the only time? Nope. Off the top of my head, there’s also 1 Timothy 6:
 for the scripture says, “You shall not muzzle an ox when it is treading out the grain,” and, “The laborer deserves his wages.”
That’s a quotation of a saying of Jesus that is also preserved in Luke 10:7.
It should be pointed out that, in the latter case, many skeptics will challenge Paul’s authorship of 1 Timothy, but the arguments that he had no hand in the letter are weak, and in any event 1 Corinthians is of undisputed Pauline authorship.
Then there are cases in which Paul does not directly quote Jesus but does directly allude to his thought.
One of these is in 1 Corinthians 7:
 To the married I give charge, not I but the Lord, that the wife should not separate from her husband
 (but if she does, let her remain single or else be reconciled to her husband) — and that the husband should not divorce his wife.
This reflects Jesus’ teaching on divorce and remarriage as found, e.g., in Mark 10:11-12 and Luke 16:18.
Note that Paul elsewhere acknowledges when he isn’t able to document something from Jesus’ teachings. Later in the same chapter, he writes:
 Now concerning the unmarried, I have no command of the Lord, but I give my opinion as one who by the Lord’s mercy is trustworthy.
And there are places where he alludes to Jesus’ teaching without making the allusion explicit (he’s trusting the reader already to know the source). An example is found in 1 Corinthians 13, where he says:
 And if I have prophetic powers, and understand all mysteries and all knowledge, and if I have all faith, so as to remove mountains, but have not love, I am nothing.
The concept of faith moving mountains is an apparent reference to a teaching of Jesus that is preserved in the Gospels (Matt. 17:20, Mark 11:23).
One could go on, but what we’ve already seen is enough to reveal how flawed are the claims that Paul never quoted or was unfamiliar with the teachings of the historical Jesus.