From Patrick Henry Reardon’s book Reclaiming the Atonement: An Orthodox Theology of Redemption (Volume 1 of 3: The Incarnate Word).
[Paul] is commonly—and not inappropriately—thought of as the Church’s earliest theologian. This persuasion, nonetheless, certainly does not mean that the Church had no theology prior to Paul’s conversion. Indeed, on the very day Ananias baptized the Apostle to the Gentiles, there already existed an authoritative body of Christian belief—a paradosis or “tradition”—of which Paul himself became both the appreciative heir and the ardent proponent. As we shall consider presently, his appeal to that authority was both prompt and insistent.
…In what forms did Paul receive this traditional information about Jesus? He received it, first of all, through the teaching ministry of the Church, beginning with the instructions he received from Ananias, the pastor of the congregation in Damascus, when he received Paul into the obedience and sacrament of faith (see Acts 9: 10–18; 22: 12–16). The living Church, this “house of the catholic obedience” (Venerable Bede’s beautiful expression), also conveyed the inherited faith to Paul through the words of her kerygmatic and catechetical material, her basic creedal forms, her hymnography, and her other prayers.
…Therefore, brethren, stand fast and hold the traditions (paradoseis) you were taught, whether by word or our epistle. (2 Thess. 2: 15)
But we command you, brothers, in the name of our Lord Jesus Christ, that you withdraw from every brother who walks disorderly and not according to the tradition (paradosin) which they received (parelabosan) from us. (2 Thess. 3: 6)
Now I praise you, brethren, that you remember me in all things and keep the traditions (paradoseis) just as I handed them on (paredoka) to you. (1 Cor. 11: 2)
I received (parelabon) from the Lord what I also handed on (paredoka) to you. (1 Cor. 11: 23)
I handed on (paredoka) to you, among the first things, that which I also received (parelabon). (1 Cor. 15: 3)
…The traditions of the Church were inseparable from the forms and content of her worship. Indeed, there is substantial evidence, from her earliest days, that the Church proceeded, at least implicitly, on the premise, “the norm of worship is the norm of belief” (lex orandi, lex credendi). The reasoning supportive of this axiom seems solid: If the Church’s prayer was an expression of her faith, then the words of the prayer must give a good idea of what the Church believed. How do we find this material?
…We don’t know how much non-liturgical poetry the earliest Christians wrote, but we do know they wrote hymns, and we know that many hymns are composed in common poetic forms. Now, if there was one thing perfectly clear about the early Christians, it was their disposition to sing the content of their faith—and not only to sing it, but to sing it together, to chant common texts they all knew by heart. Thus, we find Paul and his companions, in the dark of midnight, “praying, singing hymns (hymnoun) to God” in a Philippian jail (see Acts 16: 25). Whatever hymns they were singing, they were certainly singing them from memory.
…When we find traces of Christian hymnography in the New Testament literature, the discovery is particularly precious; in such instances we know that we are dealing with material Paul chanted before he wrote it down.