Here God clearly promises to give the grace of the Holy Spirit, that is, his abundant help, not to one or two perhaps chosen to be prophets, but to absolutely all who are worthy to receive the Spirit.
We say that this was indeed fulfilled after Christ had risen from the dead and had destroyed the power of death.
For the Spirit was given to Adam originally but did not remain in human nature, because humanity turned towards error and slipped into sin.
But when God’s only Son, though rich, chose to become poor, and being among us as a man received his own Spirit as if acquired, the Spirit remained in him.
As the evangelist John told us, this was so that the Spirit remained in him.
As the evangelist John also told us, this was so that the Spirit might dwell in us too from then on, since this time the Spirit remained in the second first-fruit of our human race, that is, Christ, who for that reason was also called the second Adam.
Therefore we must reform to become incomparably better, and to gain complete regeneration through the Spirit.
This is not the same as our original birth, I mean by earthly descent, leading us to corruption and sin (for the mind ruled by the body is death), but a second birth from above, from God through the Spirit.
For it is truly said: And they were born, not of the will of the flesh nor of the will of man, but of God.
Therefore those ranked among the children of God had to possess the grace of the Holy Spirit in abundance.
But Christ effected this also in us, as Peter’s words were to confirm:
Being therefore exalted at the right hand of the Father, and having received from the Father the promise of the Holy Spirit, he has poured out this that you see and hear.
For as man he receives from the Father what is in himself by nature. But he pours out the Spirit richly upon us, because he is God by nature, and became man.
But he also pours out the Spirit on all people. And this means not only on the Jews but on absolutely everyone called by faith, whether small or great, slave or free, non-Greek or Scythian.
For the grace of salvation in Christ is set before people all over the world, because he is the expectation of nations.
Cyril of Alexandria (c. 376-444): Commentary on Joel, 25 (PG 71:376-380); from the Monastic Office of Vigils, Wednesday of the 19th Week of Ordinary Time, Year 2.