One of the more common references many of us have heard over the years is this one: God the Father, the Son and the Holy Spirit. The tendency is to speak clearly and openly of the Father as God. The Son and Holy Spirit, well, they have a subset status. We even hear about, "He's the second person . . ." The deity test of our conviction is revealed when we stop short of saying point blank: God died. We say, "the Son of God died." This, of course, is true but it is spoken with an awkward effort to reconcile, or just as soon conceal, our poor understanding of: God is not a man that he should die. (Numbers 23:19)
God did die. He did not remain dead. The Giver of Life has demonstrated his power over death, not merely by raising the dead, but by laying down his own life to take it up again.
One of the key points that I have treasured in my understanding of my God who is Father, Son and Holy Spirit is that he is one. This is not a quantitative value as Jews, Christians and Muslims too, have feebly designated as being true of God. It is a pronouncement of unity, of agreement. What is so difficult for two human beings to be in and maintain is total, complete agreement. However, this is what God is all about: unity. Father, Son and Holy Spirit are totally and completely at one another's service to please each other, to do the will of each other.
No, neither Jew nor Muslim believe in Father, Son and Holy Spirit. Rather, the quantitative value they ascribe to the God who is one is a denial, rejection and casting away of a nagging plural form references towards God as much in the Old Testament as in the New Testament. Disciples of Jesus find themselves in similar awkwardness (needlessly) because even as we proclaim the deity of Father, Son and Holy Spirit we get tongue-tied when we declare, The Lord God is one. Father, Son and Holy Spirit may indeed, I do not know, BE heart, BE mind, BE soul separately and distinctly. These, heart, mind and soul, are the very sources of love which He called on Israel to BE towards God and neighbor and which Jesus reiterated. (Mark 12) These, heart, mind and soul, are no more to be compartmentalized and separated anymore than Father, Son and Holy Spirit.
The interesting addition of "with all your strength" by Jesus is significant. It is significant because it alludes to the flesh, the one thing counter to the nature of God who is Spirit. Yet, God was not averse or apprehensive about taking on the same human form his hands created. God took on fleshly human form. We are created in the image of God having heart, mind, and soul have been cast in vessels of flesh. Jesus reminds us that even in this flesh, the energies ("your strength") of our physical ill and well-being, is to be yet another reflection of our love towards God and towards our neighbor.