2 Our forefathers have handed down to us the tradition, that, after the Lord's ascension, when, through the coming of the Holy Ghost, tongues of flame had settled upon each of the Apostles, that they might speak diverse languages, so that no race however foreign, no tongue however barbarous, might be inaccessible to them and beyond their reach, they were commanded by the Lord to go severally to the several nations to preach the word of God. Being on the eve therefore of departing from one another, they first mutually agreed upon a standard of their future preaching, lest haply, when separated, they might in any instance vary in the statements which they should make to those whom they should invite to believe in Christ. Being all therefore met together, and being filled with the Holy Ghost, they composed, as we have said, this brief formulary of their future preaching, each contributing his several sentence to one common summary: and they ordained that the rule thus framed should be given to those who believe.
To this formulary, for many and most sufficient reasons, they gave the name or Symbol. For Symbol (κύμβολον) in Greek answers to both “Indicium” (a sign or token) and “Collatio” (a joint contribution made by several) in Latin. For this the Apostles did in these words, each contributing his several sentence. It is called “Indicium” or “Signum,” a sign or token, because, at that time, as the Apostle Paul says, and as is related in the Acts of the Apostles, many of the vagabond Jews, pretending to be apostles of Christ, went about preaching for gain's sake or their belly's sake, naming the name of Christ indeed, but not delivering their message according to the exact traditional lines. The Apostles therefore prescribed this formulary as a sign or token by which he who preached Christ truly, according to Apostolic rule, might be recognised. Finally, they say that in civil wars, since the armour of both sides is alike, and the language the same, and the custom and mode of warfare the same, each general, to guard against treachery, is wont to deliver to his soldiers a distinct symbol or watchword— in Latin “signum” or “indicium”— so that if one is met with, of whom it is doubtful to which side he belongs, being asked the symbol (watchword), he discloses whether he is friend or foe. And for this reason, the tradition continues, the Creed is not written on paper or parchment, but is retained in the hearts of the faithful, that it may be certain that no one has learned it by reading, as is sometimes the case with unbelievers, but by tradition from the Apostles.
The Apostles therefore, as we have said, being about to separate in order to preach the Gospel, settled upon this sign or token of their agreement in the faith; and, unlike the sons of Noah, who, when they were about to separate from one another, built a tower of baked bricks and pitch, whose top might reach to heaven, they raised a monument of faith, which might withstand the enemy, composed of living stones and pearls of the Lord, such that neither winds might overthrow it, nor floods undermine it, nor the force of storms and tempests shake it. Right justly, then, were the former, when, on the eve of separation, they built a tower of pride, condemned to the confusion of tongues, so that no one might understand his neighbour's speech; while the latter, who were building a tower of faith, were endowed with the knowledge and understanding of all languages; so that the one might prove a sign and token of sin, the other of faith.
But it is time now that we should say something about these same pearls, among which is placed first the fountain and source of all, when it is said—
Source: Commentary on the Apostles' Creed (New Advent)