7:1 "For whosoever does not confess that Jesus Christ has come in the flesh, is antichrist;" and whosoever does not confess the testimony of the cross, is of the devil; and whosoever perverts the oracles of the Lord to his own lusts, and says that there is neither a resurrection nor a judgment, he is the first-born of Satan.
Paul records a similar phrase in 1 Corinthians 1 (ESV):
17 For Christ did not send me to baptize but to preach the gospel, and not with words of eloquent wisdom, lest the cross of Christ be emptied of its power. 18 For the word of the cross is folly to those who are perishing, but to us who are being saved it is the power of God.
With Paul, he speaks of "the word of the cross" ("ο λογος γαρ ο του σταυρου"), whereas Polycarp urges us to "confess the testimony of the cross" ("ομολογη το μαρτυριον του σταυρου"). I find it quite natural to interpret both expressions as referencing the Gospel message, with a particular emphasis on the atoning sacrifice of Jesus. In the case of Polycarp this is especially likely, considering that he seems intent on answering the Docetic heresy.
Nevertheless, I shall now offer an alternative interpretation. Suppose Paul and Polycarp understood the cross itself to have declared a testimony to the world. While certainly a strange notion, it is not without precedent in the ancient Christian literature. Indeed, the fragmentary Gospel of the Savior personifies the cross. Consider first vv73-77, in which Jesus and his disciples continue a discourse:
73 [He] said to us, "I am among you [as] a child." 74 He said, "Amen!" 75 "A little while I am among you." 76 [...] responded, "Amen!" 77 "[Those who]
[to set (?) the world] against me [are] plotting against me because I am a stranger to it."
Thus is established a structural motif whereby Jesus (known only as "the Savior" in this Gospel) teaches a mystery, and some agent, unknown due to the missing text in v76, responds by speaking the word "Amen!" Now, we must observe that at least two of the scholars to translate this work, Hans-Martin Schenke and Stephen Emmel, believe that this agent is one or more of Jesus' disciples. Schenke suggests that v76 may read "We responded...", but Emmel declines to commit to the reading, noting that the reconstruction "He responded..." fits the text, with "he" possibly referring to the Apostle John, who also spoke in v68. I'm inclined to defer to their expert opinion, as I recommend others to do, as well. However, suppose we suspend our agreement momentarily for the purpose of considering a wilder hypothesis, that it is the cross which utters the word, "Amen!" While admittedly an unusual suggestion, consider that Jesus proceeds in later verses to speak directly to the cross. His address spans at least vv98-119, of which I provide the following excerpt:
101 "[For] those on the [right will] take shelter [under you, apart from] those on the [left, 102 O] cross, [...5 lines untranslatable...]. 103 O Cross, [...] you [...] height ... [...] for this is your desire. 104 O cross, do not be afraid! 105 I am rich. I will fill you with my wealth. 106 [I] will mount you, O cross. [I] will be [hung] upon you [...] ... [..."---"Amen!"] 107 "[...6 lines untranslatable...]. 108 [Do not] weep, O [cross], but rather [rejoice] and recognize [your] Lord as he [is coming toward you, 109 that he is [gentle] and [lowly]!"---"Amen!"
So we see that the cross is thusly personified, such that it might be said to offer its own testimony of agreement to Jesus' teachings in this Gospel.
A similar scenario is more explicitly articulated in the securer text of the Gospel of Peter, in which the author leaves no question that the cross really does speak. In the famous scene, it is very early on the morning of the Resurrection, still before dawn, and the stone has just been removed from Jesus' tomb in order to admit two young men. Frightened and disturbed, the Roman soldiers guarding the tomb run off to report the remarkable turn of events. Thus begins the following quotation:
39 And while they were relating what they had seen, again they see three males who have come out from they sepulcher, with the two supporting the other one, and a cross following them, 40 and the head of the two reaching unto heaven, but that of the one being led out by a hand by them going beyond the heavens. 41 And they were hearing a voice from the heavens saying, "Have you made proclamation to the fallen-asleep?" 42 And an obeisance was heard from the cross, "Yes."
If this account was widely known by the end of the first century, then we have a definite lead as to what specifically the cross conveyed, that Jesus preached the Gospel to the dead during the period between his own death and Resurrection. If that doctrine conflicted with Docetic theology, then Polycarp's intent is well-served by reminding his readers of it. Paul, also, in 1Co 1:18, may have wished to reference Jesus' role as an evangelist as testified by the cross.
Now, this literal interpretation of Paul's and Polycarp's "word (testimony) of the cross" is quite clearly tenuous in the extreme; I do not mean to suggest anyone take it too seriously. However, I find it to be an amusing spin on the words of Paul and Polycarp, and so I offer it up here for reflection.
 Tr. Alexander Roberts and James Donaldson. http://www.earlychristianwritings.com/text/polycarp-roberts.html
 Emmel, Stephen. "The Recently Published 'Gospel of the Savior' ('Unbekanntes Berliner Evangelium'): Righting the Order of Pages and Events." The Harvard Theological Review, Vol. 95, No. 1 (Jan., 2002), pp45-72. The translation itself is also available online: http://books.google.com/books?id=e-13kf8OPqIC
 Emmel uses a system of notation here which is best described in his own words: "I have enclosed in square brackets much that is only slightly uncertain paleographically, preferring to give the impression that some portions of the text are less securely established than is in fact the case" (Emmel, p52).
 Emmel, p58.
 Tr. Raymond Brown. http://www.earlychristianwritings.com/text/gospelpeter-brown.html