Joel's Explanation 

Apostle John who was with Christ until his last moment of the crucifixion, has revealed that Jesus Christ is not the image of Word alone, but the Image of Father, Word and the Holy Spirit, and we must believe in him to overcome this world - 5.Who is it that overcomes the world? Only the one who believes that Jesus is the Son of God. 6.This is the one who came by water and blood—Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth - 1 John 5:5-6

To understand this in a better way, apostle John first explaining the Trinity of our Heavenly Father as Father, Word and Holy Spirit - because there are three who are testifying in Heaven: the Father, the Word, and the Holy Spirit, and these three are one -  1 John 5:7

And then he explained about the Son Jesus Christ, that Jesus had all three of them(Father, Son, and the Holy Spirit) in him - and there are three who are testifying in the earth: the Spirit(Holy Spirit), and the water(Word), and the blood(Father) and the three are into the one(Jesus Christ) - 1 John 5:8

That was the reason Apostle Paul is saying that Jesus Christ is the Image of the invisible God - The Son is the image of the invisible God, the firstborn over all creation - Colossians 1:15 that means in Jesus Christ we can see the Son (The Word became flesh - John 1:14), and our Heavenly Father (Father is in me - John 10:38), and the Holy Spirit (Spirit of the Lord is upon me - Luke 4:18).


Joel's Explanation

When prophet Joel prophesied about the crucifixion of Jesus Christ, he emphasized the Trinity of Jesus Christ as Blood, Fire, and Vapor of Smoke - 30.And I will show wonders in the heavens and on the earth, blood and fire and columns of smoke. 31.The sun shall be turned to darkness, and the moon to blood, before the great and awesome day of the Lord comes. 31.And it shall come to pass that everyone who calls on the name of the Lord shall be saved. For in Mount Zion and in Jerusalem there shall be those who escape, as the Lord has said, and among the survivors shall be those whom the Lord calls - Joel 2:30-32

And Apostle Peter recalled this very prophecy in his first sermon and spoke about the Trinity of Jesus Christ - 16.But this is what was uttered through the prophet Joel: 17.And in the last days it shall be, God declares, that I will pour out my Spirit on all flesh, and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams; 18.even on my male servants and female servants in those days I will pour out my Spirit, and they shall prophesy. 19.And I will show wonders in the heavens above and signs on the earth below, blood, and fire, and vapor of smoke; 20.the sun shall be turned to darkness and the moon to blood, before the day of the Lord comes, the great and magnificent day. 21.And it shall come to pass that everyone who calls upon the name of the Lord shall be saved - Acts 2:16-21