Well the Acts of the apostles mentions the stoning of Stephen who was most likely an eye witness. We are told that Paul was involved in the killing and that a great persecution of Christians then followed in Jerusalem that caused Christians (likely many eye witnesses) to flee the city.
We have St. Paul admitting that he persecuted Christians (likely eye witnesses), and later claiming he himself was an eye witness to the risen Jesus. We also have accounts of the Apostles fearing to meet with Paul shortly after his conversion due to the threat of persecution.
Paul later tells us he did meet and talk with the apostles and that he was ready to die for Jesus, the Messiah because he himself has seen the risen Jesus.
We have the history of the Church spreading throughout the empire with universal notions of the Saints, suffering and martyrdom.
We have the gruesome persecution of Christians in Rome by Nero that must have included some eye witnesses.
We have letters from people in subsequent generations going to their deaths at the hand of Rome such as Bishop Polycarp who references the Saints that suffered before him for the faith which were eye witnesses.
We also have early historians of the Church mentioning the martyrdom of the early Christian eyewitness from the different geographic regions where Christianity grew.
Finally we have the gruesome account of the ultimate eyewitness himself - Jesus Christ whose self sacrifice in the face of death formed the base of the theology of the early Church spread by the testimony of the eye witnesses.
It could be asked that in a time of recorded persecution why would Christianity spread so quickly if the eye witnesses spreading the faith themselves by testimony shirked persecution. Wouldn’t the message fall flat if the army of eye witnesses spreading the gospel of a self sacrificing Messiah who conquered death were themselves not emulating Jesus? Would it create a culture of Christians ready to sacrifice themselves like Bishop Polycarp who knew the apostles and went to his death willingly at the hands of the Romans?
Let us therefore without ceasing hold fast by our hope and by the
earnest of our righteousness, which is Jesus Christ who took up our
sins in His own body upon the tree, who did no sin, neither was
guile found in His mouth, but for our sakes He endured all things,
that we might live in Him.
Let us therefore become imitators of His endurance; and if we should
suffer for His name’s sake, let us glorify Him. For He gave this
example to us in His own person, and we believed this.
I exhort you all therefore to be obedient unto the word of
righteousness and to practice all endurance, which also ye saw with
your own eyes in the blessed Ignatius and Zosimus and Rufus, yea and
in others also who came from among yourselves, as well as in Paul
himself and the rest of the Apostles;
being persuaded that all these ran not in vain but in faith and
righteousness, and that they are in their due place in the presence
of the Lord, with whom also they suffered. For they loved not the
present world, but Him that died for our sakes and was raised by
God for us.