Apostolic succession is one of the most thoroughly attested features of the early Church.
The earliest extra-biblical writings already assume it
St. Clement of Rome, writing to the Corinthian church around AD 96 — possibly within the lifetime of John the Apostle — directly appeals to the orderly succession of bishops appointed by the apostles as the basis for resolving the Corinthian dispute. He treats this succession as something the Corinthians already know and accept.
St. Ignatius of Antioch (c. AD 107) makes it central
In his seven letters written on the way to martyrdom in Rome, Ignatius repeatedly insists that the bishop is the visible center of the local church. "Wherever the bishop appears, there let the people be, just as wherever Jesus Christ is, there is the catholic Church" (Letter to the Smyrnaeans 8.2). Note that this is the earliest use of the word "catholic" in any Christian literature.
St. Irenaeus of Lyons (c. AD 180) makes it the test of orthodoxy
Facing Gnostic groups who claimed secret apostolic teaching, Irenaeus's response in Against Heresies Book III is that the truth of any teaching can be traced through the public, listable succession of bishops from the apostles. He gives the succession lists of multiple churches as evidence.
This was not "invented later"
The standard skeptical claim that apostolic succession was a medieval invention does not survive contact with these texts. By the 2nd century, the doctrine is universal — east, west, Egypt, Asia Minor, Italy.
For Orthodox Christians, this is not merely apologetics — it is ecclesiology
The Church is the apostolic Church not because of paperwork but because the same Spirit, the same Eucharist, and the same faith have been passed down in unbroken continuity. This is one of the four marks of the Church confessed in the Creed: "one, holy, catholic, and apostolic Church."