There were lots of contemporaries who were still alive when the various books of the New Testament were written, cherished, preserved, and distributed. For example:

For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures, and that he appeared to Cephas, and then to the Twelve. After that, he appeared to more than five hundred of the brothers and sisters at the same time, most of whom are still living, though some have fallen asleep. Then he appeared to James, then to all the apostles, and last of all he appeared to me also, as to one born out of due time. - 1 Corinthians 15:3-8

and

Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls. Acts 2:41

and then you go through counting up all the specific people who were part of the early church and widely known, for example, the last chapter of the book of Romans mentions these specific individuals that Paul greeted. It's a bit long, but that's my point - there were plenty of people in all the early churches who knew and accepted Paul.

I commend unto you Phebe our sister, which is a servant of the church which is at Cenchrea: That ye receive her in the Lord, as becometh saints, and that ye assist her in whatsoever business she hath need of you: for she hath been a succourer of many, and of myself also. Greet Priscilla and Aquila my helpers in Christ Jesus: Who have for my life laid down their own necks: unto whom not only I give thanks, but also all the churches of the Gentiles. Likewise greet the church that is in their house. Salute my well-beloved Epaenetus, who is the firstfruits of Achaia unto Christ. Greet Mary, who bestowed much labour on us. Salute Andronicus and Junia, my kinsmen, and my fellow-prisoners, who are of note among the apostles, who also were in Christ before me. Greet Amplias my beloved in the Lord. Salute Urbane, our helper in Christ, and Stachys my beloved. Salute Apelles approved in Christ. Salute them which are of Aristobulus' household. Salute Herodion my kinsman. Greet them that be of the household of Narcissus, which are in the Lord. Salute Tryphena and Tryphosa, who labour in the Lord. Salute the beloved Persis, which laboured much in the Lord. Salute Rufus chosen in the Lord, and his mother and mine. Salute Asyncritus, Phlegon, Hermas, Patrobas, Hermes, and the brethren which are with them. Salute Philologus, and Julia, Nereus, and his sister, and Olympas, and all the saints which are with them. Salute one another with an holy kiss. The churches of Christ salute you. Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them. For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple. For your obedience is come abroad unto all men. I am glad therefore on your behalf: but yet I would have you wise unto that which is good, and simple concerning evil. And the God of peace shall bruise Satan under your feet shortly. The grace of our Lord Jesus Christ be with you. Amen. Timotheus my workfellow, and Lucius, and Jason, and Sosipater, my kinsmen, salute you. I Tertius, who wrote this epistle, salute you in the Lord. Gaius mine host, and of the whole church, saluteth you. Erastus the chamberlain of the city saluteth you, and Quartus a brother. The grace of our Lord Jesus Christ be with you all. Amen.