Mark 3:14

(ubi sup.) For as a sacrament of this the children of Israel once used to encamp about the Tabernacle, so that on each of the four sides of the square three tribes were stationed. Now three times four are twelve, and in three bands of four the Apostles were sent to preach, that through the four quarters of the whole world they might baptize the nations in the name of the Father, the Son, and the Holy Ghost. It goes on: And he gave them power, &c. That is, in order that the greatness of their deeds might bear witness to the greatness of their heavenly promises, and that they, who preached unheard-of things, might do unheard-of actions.

And to send them to preach the Gospel, He gave them the power to heal diseases and to cast out demons. After He forbid the unworthy spirits to preach, He chose the saints, who would cast out the impure spirits, and they themselves would preach the Gospel with a pure mind and tongue. To whom (as the evangelist Matthew writes) He also conferred the power of healing other sicknesses, and even of raising the dead, so that the greatness of the promised heavenly things might be attested by the greatness of the deeds, and the power shown might give faith to the words, and those who preached new things might also do new things. Whence also now, when the number of the faithful has grown, within the holy Church there are many who hold to the way of virtues without having the signs of virtues. For a miracle is displayed in vain outwardly, if there is lacking the work accomplished inwardly. For according to the voice of the master of the nations, tongues are a sign not for the faithful, but for the unbelievers (1 Cor 14).

And he made it so that there were twelve with him. Certainly, for the sake of the mystery of grace, so that they might commend the salvation of the world, which they were to preach by word, also by their number. Indeed, three times four makes twelve. And three times four are the apostles sent to preach, so that they might baptize the nations in the name of the Father, and of the Son, and of the Holy Spirit through all the regions of the fourfold world. Whence it is also written concerning the holy city, Jerusalem, descending from heaven from God, that it had three gates on the east, and three gates on the north, and three gates on the south, and three gates on the west (Rev 21). Where it is shown figuratively that, with the apostles and their successors preaching, all the nations throughout the world would enter the Church in the faith of the Holy Trinity. In which sacrament as well, the children of Israel once encamped around the tabernacle, so that on every side, in a square, three tribes would remain (Num 1). For undoubtedly the early Church which was in Judea was, surrounded on all sides by believing nations from the whole world, going to set up spiritual camps to God in faith and confession of the Holy Trinity.