Polycarp's Letter to the Philippians (often simply called To the Philippians) composed around 110 to 140 AD is described by Irenaeus as follows: "There is also a forceful epistle written by Polycarp to the Philippians, from which those who wish to do so, and are anxious about their salvation, can learn the character of his faith, and the preaching of the truth." (J Stevenson, A new Eusebius [London: SPCK, 1965] p. 120) This is one quote from the epistle: "Stand fast, therefore, in this conduct and follow the example of the Lord, 'firm and unchangeable in faith, lovers of the brotherhood, loving each other, united in truth,' helping each other with the mildness of the Lord, despising no man." (Summary by Wikipedia)
