Miracles

The Holy Ghost provides sufficiently for the Church in matters profitable unto salvation, to which purpose the free graces are directed. Now just as the knowledge which a man receives from God needs to be brought to the knowledge of others through the gift of tongues and the grace of the word, so too the word uttered needs to be confirmed in order that it be rendered credible. This is done by the working of miracles, according to Mark 16:20, "And confirming the word with signs that followed": and reasonably so. For it is natural to man to arrive at the intelligible truth through its sensible effects. Wherefore just as man led by his natural reason is able to arrive at some knowledge of God through His natural effects, so is he brought to a certain degree of supernatural knowledge of the objects of faith by certain supernatural effects which are called miracles. Therefore the working of miracles belongs to a free grace.

Just as the prophet's mind is moved by divine inspiration to know something supernaturally, so too is it possible for the mind of the miracle worker to be moved to do something resulting in the miraculous effect which God causes by His power. Sometimes this takes place after prayer, as when Peter raised to life the dead Tabitha (Acts 9:40): sometimes without any previous prayer being expressed, as when Peter by upbraiding the lying Ananias and Saphira delivered them to death (Acts 5:4,9). Hence Gregory says (Dialog 2:30) that "the saints work miracles, sometimes by authority, sometimes by prayer." In either case, however, God is the principal worker, for He uses instrumentally either man's inward movement, or his speech, or some outward action, or again the bodily contact of even a dead body.

~ St. Thomas Aquinas