In The Good God, Mike Reeves writes briefly on the problem of evil. A unitarian God would either be threatened by evil or the author of it, but...
The triune God... is the sort of God who will make room for another to have real existence. The Father, who delights to have a Son, chooses to create many children who will have real lives of their own, to share the love and freedom he has always enjoyed. The creatures of the triune God are not mere extensions of him; he gives them life and personal being. Allowing them that, though, means allowing them to turn away from himself – and that is the origin of evil. By graciously giving his creatures the room to exist, the triune God allows them the freedom to turn away without himself being the author of evil. The Good God, p39-40
This is so helpful in placing "evil" in the context of trinitarian thought. Let's take this thought a step further and consider freedom also in triune terms.
Because actually the Christian does not think of "freedom" the same way as the unitarian. Or at least we shouldn't!
Often, however, we do conceive of "divine sovereignty" in unitarian terms. At that point "human freedom" is considered as, ultimately, a fiction or as an escape from God's all-determining supremacy. If we imagine God's sovereignty as uni-directional then it can only bear down on that which is other than God. In this case God is always determining. That which is other than God is always being determined.
But what if the Lord has existed in I-Thou relations in eternity? What if there's been reciprocity and mutual-determination within God's being? And what if, in the determination of this God, the Son becomes Man to draw the creature into these mutual relations?
Well you start to see give-and-take, offer-and-response as something that doesn't threaten God's divine nature, but that actually constitutes it!
Freedom, then, is not something opposed to divine sovereignty. Freedom is finding your place as your distinct self in these relationships.
It's our distinctness that Mike is highlighting in the quote above. We do not originate as growths within the divine being. We are given a concrete and particular existence outside of God. Our freedom therefore speaks of our genuine other-ness to God. But we must always say that this otherness is intended as an otherness-in-relation.
Think about it like this: the Son is definitionally free (since He is "the Son" and not "A Slave"). But "Son" also speaks of "Obedience" "Likeness" "Sent-ness". His freedom is found in relationship with His Father - He is who He is in that union.
The same will be true for our freedom. We are set free by the Son (John 8:36) - liberated into His Sonship (Galatians 4:4-7). Therefore it is very much a freedom found in the triune relationships - united to the Son, filled with the Spirit of Adoption, calling on our Abba, Father. To be outside these relationships is not freedom, it's slavery.
Therefore we mustn't define things in such a way that sinning is considered an expression of freedom. Choosing to reject God is not the exercise of freedom but its opposite. Rejecting this God means embracing slavery.
Therefore freedom is not centred on the garden of Eden. It's bed-rock definition is not 'our ability to choose evil.' For a start, that places our freedom above God, and above the freedom of the new creation! No, as Mike well knows, the freedom he mentions in this quote - i.e. the freedom of "allowance" and "distinctness" - is not the whole story. We need to go to another garden to find a true definition of freedom.
In Gethsemane the Son submits His will to His Abba, Father to save us slaves who chose the darkness. And in this submission He expresses His nature as "Son" more clearly than ever. Here is freedom - here is Man living responsibly before His God and expressing His true identity. But it's dripping in the blood, sweat and tears of submission and sacrifice.
All of this is to say that "freedom" does indeed entail God's allowance of man to turn. But it's in no way exhausted or defined by that possibility. True freedom is upheld by this: when we turned to the darkness, God did not prevent us but pursued us. As the name implies, it's redemption rather than creation that makes us free. It's ultimately in His decision and act that we find freedom.