Objection 1. It seems that the second beatitude, "Blessed are the meek," does not correspond to the gift of piety. For piety is the gift corresponding to justice, to which rather belongs the fourth beatitude, "Blessed are they that hunger and thirst after justice," or the fifth beatitude, "Blessed are the merciful," since as stated above (1, Objection 3), the works of mercy belong to piety. Therefore the second beatitude does not pertain to the gift of piety.
Objection 2. Further, the gift of piety is directed by the gift of knowledge, which is united to it in the enumeration of the gifts (Isaiah 11). Now direction and execution extend to the same matter. Since, then, the third beatitude, "Blessed are they that mourn," corresponds to the gift of knowledge, it seems that the second beatitude corresponds to piety.
Objection 3. Further, the fruits correspond to the beatitudes and gifts, as stated above (I-II, 70, 2). Now among the fruits, goodness and benignity seem to agree with piety rather than mildness, which pertains to meekness. Therefore the second beatitude does not correspond to the gift of piety.
On the contrary, Augustine says (De Serm. Dom. in Monte i): "Piety is becoming to the meek."
I answer that, In adapting the beatitudes to the gifts a twofold congruity may be observed. One is according to the order in which they are given, and Augustine seems to have followed this: wherefore he assigns the first beatitude to the lowest gift, namely, fear, and the second beatitude, "Blessed are the meek," to piety, and so on. Another congruity may be observed in keeping with the special nature of each gift and beatitude. On this way one must adapt the beatitudes to the gifts according to their objects and acts: and thus the fourth and fifth beatitudes would correspond to piety, rather than the second. Yet the second beatitude has a certain congruity with piety, inasmuch as meekness removes the obstacles to acts of piety.
This suffices for the Reply to the First Objection.
Reply to Objection 2. Taking the beatitudes and gifts according to their proper natures, the same beatitude must needs correspond to knowledge and piety: but taking them according to their order, different beatitudes correspond to them, although a certain congruity may be observed, as stated above.
Reply to Objection 3. In the fruits goodness and benignity may be directly ascribed to piety; and mildness indirectly in so far as it removes obstacles to acts of piety, as stated above.
Source: Summa Theologica (New Advent)