6 Let us beware of such dangerous temptations of pride, and let us rather consider the fact that the Apostle Paul himself, although stricken down and admonished by the voice of God from heaven, was yet sent to a man to receive the sacraments and be admitted into the Church; and that Cornelius the centurion, although an angel announced to him that his prayers were heard and his alms had in remembrance, was yet handed over to Peter for instruction, and not only received the sacraments from the apostle's hands, but was also instructed by him as to the proper objects of faith, hope, and love. And without doubt it was possible to have done everything through the instrumentality of angels, but the condition of our race would have been much more degraded if God had not chosen to make use of men as the ministers of His word to their fellow-men.
For how could that be true which is written, “The temple of God is holy, which temple you are,” if God gave forth no oracles from His human temple, but communicated everything that He wished to be taught to men by voices from heaven, or through the ministration of angels? Moreover, love itself, which binds men together in the bond of unity, would have no means of pouring soul into soul, and, as it were, mingling them one with another, if men never learned anything from their fellow-men.
Source: Christian Doctrine (New Advent)