The prophet Joel is drawing upon imagery and concepts from Ezekiel (who had in turn drawn from his predecessor Jeremiah), and the following paragraphs will help to tie the thoughts all together.
First, the Day of Pentecost happened to be the same day when the Law of Moses was given on Sinai (Feast of Weeks). That is, the Book of Acts indicates that the New Covenant was given to Israel on the exact same day that the Old Covenant was given to Israel (Feast of Weeks = Pentecost).
As regards the New Covenant, there are three primary aspects to the New Covenant according to Hebrews 8:8-12, which is a quotation of Jeremiah 31:31-34...
- The forgiveness of sins
- The placement of God's law in the mind, and the writing of God's law on the heart
- The universal knowledge of the Lord
In Hebrews 10:11-18, we read that "at the present time" only the first two aspects of the New Covenant are in effect with one slight change. That is, instead of the placement of God's law in the mind and the writing of God's law on the heart, the law of God is instead written on the mind and is placed upon the heart. In other words, at the current time there is an emphasis and priority in the New Testament on the renewal of ones mind through the written word of God (Rom 12:1-2 and Eph 4:22-24).
Secondly, in the Book of Ezekiel the New Covenant appears to include the new heart, which amplifies what Jeremiah had earlier mentioned was the second aspect of the New Covenant.
Ezek 11:19-20 (NASB)
19 And I will give them one heart, and put a new spirit within them. And I will take the heart of stone out of their flesh and give them a heart of flesh, 20 that they may walk in My statutes and keep My ordinances and do them. Then they will be My people, and I shall be their God.
Ezekiel is beckoning the reader to the Book of Deuteronomy, where the Lord had first made this promise that he would one day remove their hearts of stone.
Deut 30:6 (NASB)
6 “Moreover the Lord your God will circumcise your heart and the heart of your descendants, to love the Lord your God with all your heart and with all your soul, so that you may live...”
In other words, Ezekiel is prophesying that the Lord will not only circumcise their hearts (by the removal of the diseased heart of stone), but will also provide his people the "new" heart, which will enable them to obey him.
For example, Ezekiel mentions the new heart yet a second time in Ezekiel 36:24-28 (which hearkens the reader yet again back to Deut 30:6). But in this latter passage of Ezekiel, Ezekiel amplifies yet more on this new heart and now mentions both water and the giving of God's Spirit in the same context. That is, there is now the image of water raining down ("sprinkling") from heaven with the giving of the Spirit of God, and this imagery is what Joel (and then Peter) had used to refer on the Day of Pentecost, when the New Covenant was announced to Israel (which was the same day that the Old Covenant was announced to Israel).
In other words, Ezekiel amplifies the second aspect of Jeremiah's New Covenant, when he ties the circumcised heart to the Book of Deuteronomy. Not only does Ezekiel go back to Deuteronomy, but indicates that the good news is that a new heart will be provided with both "water" and and God's Spirit.
So, if we connect Jeremiah's New Covenant with Ezekiel's amplification of the new heart (the second aspect of Jeremiah's New Covenant), we then arrive at four primary aspects of the New Covenant--
- The forgiveness of sins
- The placement of God's law in the mind, and the writing of God's law on the heart
-- The old heart is first excised and replaced with a new heart
- The giving of "water" and the Spirit of God
- The universal knowledge of the Lord
So to recap, while Ezekiel does not mention Jeremiah's New Covenant per se, he does mention the heart of stone, which is not only excised by circumcision, but is then replaced with a new heart. Both the "sprinkling of water" (which Ezekiel says will clean the "filthiness") and the Spirit of God (who will enable obedience of the new heart) are part of this package.
It is therefore interesting, that if this water is eternal life which Jesus had described in John 4:13-14, then the filthiness is not sin, but spiritual death. That is, it is blood that removes sins (Lev 17:11 or Heb 9:22), but only water was necessary for the removal of any uncleanliness associated with death (Lev 15; Lev 17; Lev 22; Num 19; etc.). The removal of the old heart in the Christian New Testament is therefore the circumcision of spiritual death by the Spirit of God (Colossians 2:11-12 compared to Romans 2:29).
In other words, the outpouring from heaven in Acts 2:28-29 was the baptism of the Spirit of God (or "washing" in 1 Pet 3:21 and Tit 3:5), which was the inauguration of the New Covenant in full force (like a downpour of rainwater from heaven). This baptism of the Holy Spirit however was dry, because the "water" was eternal life, which was poured out by, and through, God's Spirit. (Spiritual death was thus cleansed away.) The fire of tongues takes us yet back again to Sinai, when the words of God were written on tablets of stone on the Feast of Weeks. On Pentecost (which is the same day as the Feast of Weeks) that same fire was now putting the word of God not on tablets of stone, but was now being written on minds and hearts according to 2 Corinthians 3:3. The audible, dynamic and intelligible articulation of the word of God resulted.
Therefore the prophet Joel, while he had never made mention of the "New Covenant" per se, had nonetheless drawn upon imagery and concepts from Ezekiel. Thus on Pentecost Peter quoted from Joel in Acts 2:28-29 regarding the "outpouring" of the Spirit (described by Ezekiel), but the context was the the New Covenant (described by Jeremiah), since the day of Pentecost was in fact the day that the Old Covenant was given to Israel.