Incompletely human

Even if humans were statistically just as likely to sin as they are to love their children, it cannot, for Thomas, be right in the same sense of the word “natural” to say that human beings naturally sin as that they naturally love their children. To find oneself puzzled by the words of the Letter to the Hebrews because the exception of his sinlessness seems a very drastic limitation upon Christ's solidarity with us; to find oneself wondering whether Christ could really have known what it is like to be us if he does not experience sin, as it were, from the inside; to imagine that one of the solidarities in which human beings share is solidarity in sin because we all do it and so understand one another in our sinning; to conclude, therefore, that it is a restriction on Christ's solidarity with us that he is absolutely sinless, for a Christ who could not sin would seem not to share our human nature in its concrete actuality—all this is, for Thomas, getting things upside down. 

...Unlike Christ, it is we who are incompletely human. Docetism is the heresy that Jesus is not truly a human being, but only apparently so, because he is too good to be true. Thomas's position is that it is we who, on account of our sinfulness, are not good enough to be true. We do not know for ourselves and from ourselves what it is like to be truly human. From ourselves and from others at best we get partial glimpses of what a true human being looks like. But we have no full picture of what it is to be truly human except from the human nature of Christ. The point about the exception made in the Letter to the Hebrews is that “solidarity in sin” is in fact an oxymoron.

...the casual theological identification of fallen human nature with human nature as such ignores what is manifestly intended by the author of Hebrews, for whom Christ alone can perfectly communicate with us because he is perfectly, indefectibly, human; it is precisely because he is free of sin that the intimacy of Christ's solidarity with our sinful selves is unhindered, and in him the view of what we truly are is uncluttered by the obstructing impedimenta of sin; and so it is precisely because we are dehumanized by our sinfulness that our solidarities with Christ and with one another are so tragically awry, and in consequence, our relationships, personal and political, beset by fantasy and illusion. For Thomas, Christ is more intimately human than we are, not less; closer to our true selfhood than we are; in closer solidarity with us because he is without sin, not distanced from us on account of his innocence, as we are distanced from ourselves by sin, from one another, and from God.

From Denys Turner's Thomas Aquinas, a Portrait.