Philip Schaff in “History of the Christian Church” Vol. II §27 noted that the early catacombs contained inscriptions where the departed are asked to pray for their living relatives (p. 83). What is interesting is a letter from the Church of Smyrna dated AD 155:

“Him indeed we adore (προσκυνουμεν) as the Son of God; but the martyrs we love as they deserve (αγαπωμεν αξιως), for their surpassing love to their King and Master, as we wish also to be their companions and fellow-disciples” (pp. 82-83).

The distinction between the worship of Christ and the veneration of the saints is very much the same distinction Orthodox Christians use today. This shows the remarkable continuity of Orthodoxy with early Christianity.

It can also be taken as evidence that the distinction between adoration and veneration was not concocted by the Seventh Ecumenical Council but has very early roots.