Some additional color and continuation of snipnits from the larger article posted above:
Early Extra-Biblical Sources Affirming Paul’s Apostleship
Ignatius of Antioch (A.D. 35-110)
Ignatius of Antioch was a 1st century pupil of the original Apostles.(23) This is important because if Paul was a false teacher and usurper, Ignatius, being a follower of the Apostles and their Gospel (he often quoted the Gospels of Matthew and John as well), would have pointed out Paul’s supposed theological errors or commented on Paul being a supposed false Apostle. However, this 1st century martyr Bishop offers early data in support of Paul’s association with the other Apostles as well as Paul’s rightful authority in the church. Ignatius wrote the following in A.D. 110 to the Christians in Rome:
“I do not command you, as Peter and Paul did.”(24)
This extremely early material is affirming that Paul worked alongside Peter in leading and commanding the Christian church in Rome. Ignatius has other valuable remarks affirming the reliability of the Apostle Paul. For example, in writing to the Christians in Ephesus Ignatius relays that Paul accurately gave the Gospel to the Ephesians, that Paul was martyred for his faith (which also shows Paul’s reliability) as well as his deep respect and honor for Paul:
“You are initiated into the mysteries of the Gospel with Paul, the holy, the martyred, the deservedly most happy, at whose feet may I be found, when I shall attain to God; who in all his Epistles makes mention of you in Christ Jesus.”(25)
We know that Ignatius was fed to lions in a Roman coliseum for his faith since Christianity was being persecuted by the Roman state.(26) This shows that Ignatius so firmly believed in his theology (which included Paul as a true Apostle with inspired doctrine) that he was willing to be martyred for it. If he knew Paul was an imposter or deceiver he would not be willing to be martyred for his faith. As the saying goes “liars make poor martyrs.”
Clement of Rome (A.D. ?-101)
Clement of Rome was a 1st century Christian secretary of the church at Rome responsible for correspondence with other churches.(27) There is also evidence to suggest that he was a prominent presbyter of the Roman church. Some believe he was the “fellow worker” Paul mentioned in Philippians 4:3. In his work Against Heresies chapter 3, book 3, section 3 Irenaeus, the 2nd century early writer, notes that Clement of Rome knew the original Apostles:
"...after him, in the third place from the apostles, Clement was allotted the bishopric. This man, as he had seen the blessed apostles, and had been conversant with them, might be said to have the preaching of the apostles still echoing [in his ears], and their traditions before his eyes. Nor was he alone [in this], for there were many still remaining who had received instructions from the apostles."
In his letter The First Epistle of Clement also known as First Epistle to the Corinthians written in A.D. 96 Clement states the following about Paul:
“Owing to envy, Paul also obtained the reward of patient endurance, after being seven times thrown into captivity, compelled to flee,and stoned. After preaching both in the east and west, he gained the illustrious reputation due to his faith, having taught righteousness to the whole world, and come to the extreme limit of the west, and suffered martyrdom under the prefects. Thus was he removed from the world, and went into the holy place, having proved himself a striking example of patience.”(28)
“Take up the epistle of the blessed Apostle Paul. What did he write to you at the time when the gospel first began to be preached? Truly, under the inspiration of the Spirit, he wrote to you concerning himself, and Cephas, and Apollos, because even then parties had been formed among you.”(29)
Notice that Clement, in representing the beliefs of the 1st century Church at Rome, grants Paul’s reliability. He mentions Paul’s labours for the Gospel, his persecution for the faith, and his martyrdom. He states that Paul was a “striking example of patience” or in other words “endurance.” Notice also in the second citation that Clement attests to Paul’s reliability in that he calls him a "blessed Apostle," takes Paul’s epistle to the Corinthians as authoritative and valid with respect to Gospel truth, and states that Paul wrote his letter “under the inspiration of the Spirit.” This means Clement, and subsequently those in the 1st century Church of Rome, believed Paul’s letters to be inspired God-breathed Scripture - canon.
Polycarp of Smyrna (A.D. 69-155)
Polycarp was a 1st century Bishop like Ignatius. He was also a student or pupil of John and the other Apostles.
Therefore, in light of all of this early evidence which demonstrates that Polycarp knew the original Apostles, knew their original 1st century Gospel message, was appointed Bishop of Smyrna by the Apostles and suffered brutal martyrdom for his faith, it is indeed interesting that he would then affirm the Apostle Paul as genuine and sound theologically if Paul was a false Apostle. Polycarp states:
“For neither I, nor any other such one, can come up to the wisdom of the blessed and glorified Paul. He, when among you, accurately and steadfastly taught the word of truth in the presence of those who were then alive. And when absent from you, he wrote you a letter, which, if you carefully study, you will find to be the means of building you up in that faith which has been given you, and which, being followed by hope, and preceded by love towards God, and Christ, and our neighbour, is the mother of us all.”(37)
“I exhort you all, therefore, to yield obedience to the of righteousness, and to exercise all patience, such as you have seen [set] before your eyes, not only in the case of the blessed Ignatius, and Zosimus, and Rufus, but also in others among yourselves, and in Paul himself, and the rest of the apostles.”(38)
“For if a man cannot govern himself in such matters, how shall he enjoin them on others? If a man does not keep himself from covetousness, he shall be defiled by idolatry, and shall be judged as one of the heathen. But who of us are ignorant of the judgment of the Lord? Do we not know that the saints shall judge the world? as Paul teaches. But I have neither seen nor heard of any such thing among you, in the midst of whom the blessed Paul laboured, and who are commended in the beginning of his Epistle. For he boasts of you in all those Churches which alone then knew the Lord; but we [of Smyrna] had not yet known him.”(39)
If Paul was an imposter, then Polycarp, knowing John and the other Apostles as well as their orthodox theology, would have spoken out against Paul. On the other hand if someone asserts that Polycarp was a liar or conspirator trying to mislead people to follow Paul for some nefarious absurd reason then Polycarp would not willingly go to his death for his faith. This evidence is a fatal blow to the egregious falsehood of anti-Pauline critics.
It's a good read.