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The simplest answer is that denying the Trinity diminishes the person of Jesus. There is an enormous gap between saying that Jesus is the Almighty God and saying that he is one of God's creations. Even if Jesus was God's first and greatest creation, he would still be much less awesome than if he were God himself. So, it is understandable that trinitarians would be offended by someone who degrades Jesus from God to a mere creation. The person of Jesus Christ is, obviously, central to the Christian faith, so to differ in such a large way on such a central belief is, in the mind of many, the height of heresy.
Secondarily, trinitarians often believe that the sacrifice of Jesus on the cross for our sins was only effective because he was God. People holding to this view would argue that salvation for the world could not be achieved by the death of a mere man, even if he was a sinless man. People holding to this view would therefore be further offended by a unitarian* because, in their mind, the unitarian would be endangering the souls of anyone that he might convince with his heresy.
* In the lower-case, historical sense of the word (i.e. non-trinitarian)
Upvote:0
Trinitarians believe that God himself is a very emotional being - an overwhelmingly joyful being in fact. Eric's answer to Why did God create the Heavens and the Earth? gives some quotes from John Piper's book Desiring God:
In creation, God “went public” with the glory that reverberates joyfully between the Father and the Son. There is something about the fullness of God’s joy that inclines it to overflow. There is an expansive quality to His joy. It wants to share itself. The impulse to create the world was not from weakness, as though God were lacking in some perfection that creation could supply. “It is no argument of the emptiness or deficiency of a fountain, that it is inclined to overflow.”
God loves to behold His glory reflected in His works. So the eternal happiness of the triune God spilled over in the work of creation and redemption. And since this original happiness was God’s delight in His own glory, therefore the happiness that He has in all His works of creation and redemption is nothing other than a delight in His own glory. This is why God has done all things, from creation to consummation, for the preservation and display of His glory. All His works are simply the spillover of His infinite exuberance for His own excellence. (Desiring God, p44)
So we experience and share in God's joy because he is totally overflowing with joy. The reason why he is so joyful is because of he is eternally happy with himself - each person in the Trinity is constantly pleasing the other two.
A non-trinitarian God would be very different, and his character would be very different too. I won't say that there aren't any non-trinitarian explanations for God's joy, but I haven't heard any either.
Many trinitarian Christians would rightly get upset at the idea of a non-trinitarian God because they believe that the only source of their own joy is from God's own overflowing joy.
Upvote:5
In addition to Steven Doggart's answer (with which I wholeheartedly agree) I suggest that there is considerable emotional appeal in the idea that God himself would voluntarily give up his position, enter this fallen world with all the frailties of a human and sacrificed all for his lost and rebellious creation. As trinitarians we believe in a God who understands our situation through having experienced it for himself, whilst at the same time living in a way that exemplifies both the character of God and the depths of our fall.
In contrast, non-trinitarians appear to depict a God who is far more distant from his creation, a God who is willing to leave the means of our salvation in the hands of a servant, albeit a noble and worthy one.