How do Protestants answer the Catholic accusation of historicity? The Roman Catholic Church has a claim of apostolic succession, from St. Peter transited by the laying on of hands.
Most Protestants reject this understanding of apostolic succession, and instead teach that apostolic succession simply means where &quot;the Word and the Sacraments are purely administered and taught.&quot;
However, the Catholic accusation of historicity still stands.
So how do Protestants explain/justify the fact that their understanding of Christianity only started in the XVI century? Was the whole Church corrupt or in error till then? If not, how come, since there was only the Orthodox and Catholic Churches (+ others but not Protestants)?
Protestant reformers were at pains to argue that all they sought to do was restore the church to purity which had become corrupted in medieval times. They wanted to ensure that their teachings were not just in line with Scripture but with many of the church fathers e.g. Augustine. You will find that the writings of the reformers are full of quotes to many early church fathers.
Just to give one example, the first homily from the first Anglican book of homilies (&quot;A fruitful exhortation to the reading and knowledge of Holy Scripture&quot;) quotes seven times from ancient writers - Chrysostom, Augustine, and Fulgentius.
John Calvin says this in his reply to Cardinal Sadeleto:

But here you bring a charge against us. For you teach that all which has been approved for fifteen hundred years or more, by the uniform consent of the faithful, is, by our headstrong rashness, torn up and destroyed. Here I will not require you to deal truly and candidly by us, (though this should be spontaneously offered by a philosopher, not to say a Christian.) I will only ask you not to stoop to an illiberal indulgence in calumny, which, even though we be silent, must be extremely injurious to your reputation with grave and honest men. You know, Sadolet, and if you venture to deny, I will make it palpable to all that you knew, yet cunningly and craftily disguised the fact, not only that our agreement with antiquity is far closer than yours, but that all we have attempted has been to renew that ancient form of the Church, which, at first sullied and distorted by illiterate men of indifferent character, was afterwards flagitiously mangled and almost destroyed by the Roman Pontiff and his faction. [My emphasis]

So the Protestant reformers did not believe their understanding of Christianity only started in the 16th century. They believed they were doing away with the corruptions of the faith which had come into the church in medieval times to a more pure Scriptural faith. They quoted from the Fathers to support what they were saying.