Peter, Linus, Cletus, Clement: the line of the First Century popes; you might recognize them from Eucharistic Prayer I.
Possibly mentioned in Philippians 4:3 and later by Jerome and Origien, there is little that we know of Clement except for his letter to the Corinthians which has survived as the earliest Patristic (non-Scriptural) writing that we have. What we can know is that it does show a concern for the larger Church by the bishop of Rome, if anyone wants to be part of that fight.
We speak about a Roman Catholic Church, or a Greek Orthodox Church, or any number or 'Rite' Churches, but Clement shows us the catholic nature of the Church that we speak of in our shared Creed. What he tells the Corinthians is nothing surprising or new and sounds very Pauline to me. What the letter does do is continue the Apostolic leadership that we speak of, whether talking about a bishop, a metropolitan, a patriarch, or a pope within a catholic Church. We each, in our own traditions, have much to share and teach one another...all we have to do is start listening in the love that Clement so eloquently speaks about.
Let him who has love in Christ keep the commandments of Christ. Who can describe the [blessed] bond of the love of God? What man is able to tell the excellence of its beauty, as it ought to be told? The height to which love exalts is unspeakable. Love unites us to God. Love covers a multitude of sins. Love bears all things, is long-suffering in all things. There is nothing base, nothing arrogant in love. Love admits of no schisms: love gives rise to no seditions: love does all things in harmony. By love have all the elect of God been made perfect; without love nothing is well-pleasing to God. In love has the Lord taken us to Himself. On account of the love He bore us, Jesus Christ our Lord gave His blood for us by the will of God; His flesh for our flesh, and His soul for our souls.
You see, beloved, how great and wonderful a thing is love, and that there is no declaring its perfection. Who is fit to be found in it, except such as God has vouchsafed to render so? Let us pray, therefore, and implore of His mercy, that we may live blameless in love, free from all human partialities for one above another. All the generations from Adam even unto this day have passed away; but those who, through the grace of God, have been made perfect in love, now possess a place among the godly, and shall be made manifest at the revelation of the kingdom of Christ. For it is written, "Enter into your secret chambers for a little time, until my wrath and fury pass away; and I will remember a propitious day, and will raise you up out of your graves." [Isaiah 26:20] Blessed are we, beloved, if we keep the commandments of God in the harmony of love; that so through love our sins may be forgiven us. For it is written, Blessed are they whose transgressions are forgiven, and whose sins are covered. Blessed is the man whose sin the Lord will not impute to him, and in whose mouth there is no guile. This blessedness comes upon those who have been chosen by God through Jesus Christ our Lord; to whom be glory for ever and ever. Amen.
-- Letter to the Corinthians 49-50