At the heart of it is the article of faith held by Christians that Jesus was raised from the dead, and ascended into Heaven. This is the single most relevant point of the whole of Christianity. For as Paul says to the Corinthians
If there is no resurrection of the dead, then neither has Christ been raised.So this is little more than an effort to undermine the very cornerstone of our faith.
And if Christ has not been raised, then empty [too] is our preaching; empty, too, your faith.
Now the bible gives no proof of Christ's resurrection. True, there are eye witness accounts of the results; The empty tomb. The women, who were the first to be told. The Apostles. But these are second hand accounts. Peter never writes of his actual experiences at the tomb, or even, in detail of his talks with the Resurrected Christ. Paul never saw the empty tomb, that we know of. Paul did not see the Resurrected Jesus until after his Ascension.
That makes the Resurrection a matter of Faith. We believe the Resurrection took place. Either because we have been told so by people we trust, the Evangelists, our Catechism teacher, the teaching authority of the Church , or because the Holy Spirit has spoken to us, as it did to Peter, when he perceived Jesus as the Messiah.
Now reason also helps us in this regard. Of the Eleven, ten were martyred, most in very painful, horrendous ways. It's hard to believe that they would have went to their deaths thusly for something they made up. If there had been a body why wouldn't the Romans or the Sanhedrin have discredited these pesky Christians by producing the body?
Faith informed by reason, practically the mantra of the Catholic Church.