The "Jesus is just Horus / Mithras / Osiris recycled" claim went viral with Zeitgeist (2007) and has been debunked by actual historians of religion ever since.
What the primary sources actually say
Horus was not born of a virgin (Isis used magic on the dead Osiris's body), did not have 12 disciples, was not crucified, and was not resurrected on the third day. Almost every parallel claimed in popular memes is fabricated or distorted.
Mithras (the Roman mystery cult, not the Persian deity) flourished in the 2nd century A.D. — after Christianity. Where there is overlap, the influence runs the other direction.
Dionysus, Osiris, Tammuz were "dying and rising gods" in a much weaker sense: vegetation cycles, repeated annually. None of them was a specific historical person executed under a named governor in a named year, witnessed publicly to have risen bodily.
The Christian difference
The Resurrection is anchored in history: under Pontius Pilate (named in the Creed precisely because the event is dated), in Jerusalem, witnessed by hundreds, attested in documents written within decades by people willing to be killed for the claim. The historian Bart Ehrman — no friend of orthodoxy — grants that the earliest disciples genuinely believed they had seen the risen Jesus.
The Orthodox lens
C.S. Lewis (whose Christian conversion partly turned on this point) argued that the pagan myths were not refutations of Christianity but anticipations: humanity dreaming, with some accuracy, of what God would actually do. "The story of Christ is simply a true myth: a myth working on us in the same way as the others, but with this tremendous difference — that it really happened."
The Fathers — Justin Martyr, Clement of Alexandria — said much the same: the logos spermatikos, seeds of the Word, were scattered through human culture, all pointing forward to the Word made flesh.
A pastoral word
If you encountered the "Jesus is just a recycled myth" claim online and it shook you, take heart — it doesn't survive a single careful trip to primary sources. The Resurrection is uniquely public, uniquely historical, and uniquely yours.