The Bible refers to the substitutionary death of Christ on behalf of the sinner as a debt that must be paid to satisfy God's justice against sin and the sinner. A just God (that is who he is along with all his attributes, not just something he possess) cannot simply overlook sin without violating his very essence of holy. It can be temporarily covered as to final judgment as is illustrated for us in the animal sacrifices for those in a covenant relationship. But even then, it was a substitute of death for temporary life.
In looking at the magnitude of the debt, and the condition of humanity as sinners by nature, we can see why Jesus must be fully human and fully God. and we can see why the Holy Spirit must also be deity through the work that Scripture says he does---first in regeneration (John 3; John 1).
We can only begin to grasp the height from which we fell, and the necessity of God himself to change that condition, when we begin to grasp the holiness of God. And the holiness of God can only be grasped by contemplating him (not us) in his word, and across all sound theological doctrines. Focused, intentional, looking, at his actions of both blessing and judgement that reveal who he is. By not looking at him as a being composed of attribute parts but as a being who is those attributes as essence. Here too we will begin to see that he is a trinity of Father, Son, and Holy Spirit, for Scripture shows all three as participants in redemption of sinners.
From that perspective we can begin to see the height from which we fell as creatures having been created in his image and likeness, and how great our debt is. How impossible it is for another creature, no matter how "good" they are, to accomplish what Christ accomplished on the cross by paying that debt for us on his human body. A creature, a created human being would first have to pay for himself by virtue of being created/born in Adam, even if he had no sin of his own. His value as a created human would simply not have the holy value to accommodate the debt owed. It would not have the power to do so because it would not possess the power of God required to defeat sin and death. Only God possesses that power and only God has that prerogative. He cannot give it to another, as some unitarian thought claims, or he would lessen himself.
The Holy Spirit too, must be personal and deity because Scripture presents him as "person". Personal pronouns as to his actions. We see him in Gen 1 as hovering over the face of the waters, and darkness was over the face of the deep. God said let there be light and there was light. The Holy Spirit producing the light at Gods request. In verse 26 God said, "let us make man---", plural. In John 1:3,10; Col1:16; 1 Cor 8:6 Jesus is creator. Which would mean that the one Jesus calls Father (God) are there together in the beginning creating. There are three involved in doing what only God can do. The Holy Spirit is distinctly named in the creation account. He is distinctly named in redemption.
The Holy Spirit as distinct is shown taking certain types of action throughout Scripture and is shown as the one who brings about the new birth, and the one who seals and indwells the believer. Doing things only God can do. All three persons of the Trinity, all equal in essence of deity are absolutely necessary in redemption.
