To start with he had that bodily chastity which we usually identify with purity though often not realising why the Christian mind attaches such importance to it. If we are to appreciate its worth we should perhaps envisage it not only in itself but in what it makes possible, or what its opposite prevents. In reality, purity understood in the narrow and ordinary sense as chastity of body, imagination and desire, involves the whole spiritual life, because it opens — and its opposite closes — every possible relationship with God. Consider a soul that is crude and impure: it will not deny the teachings of the faith, but it becomes hardened and ends by shutting itself off front the finest elements of that faith; its spiritual sensitivity dulls; it will not reject the central dogmas, but their noblest content no longer interests it: the Virgin, the angels, the sacraments, the religious life, contemplation; it comes to assert that nothing can be known about them and that they do not exist. A pure soul, on the contrary, instinctively opens itself out to these things; it experiences them to the extent in which it is sensitively pure. It would seem that faithfulness on this point opens its eyes and provides the evidence which crudity would instantaneously conceal. It realises with certainty that its development is an immediate relationship with the sensitivity and fidelity of its life.
We can now have a somewhat clearer idea of what purity, bodily purity, meant for St Thomas in the work of service as a theologian to which he was called, and we can see to what extent it was a positive element in his soul as a servant of the truth. We need hardly stress here the famous scene of his temptation — it has been in any case a little touched up — or the testimony given after his death by his most intimate friend that he had always preserved the purity of a child.
There is, however, a deeper aspect of his purity as a servant of the truth; it is that of the inner purity of his soul, in the most positive and fullest sense of that word. As a correlative to poverty considered as a fundamental attitude of the soul, this purity of the servant of the truth consists in allowing no admixture of self to enter into this truth, no toning down, no rejection, but instead a complete self-surrender to its demands and an acceptance of the Other, the Master, as he really is in himself and not as we might have imagined him to be. It is he, the Master, who must be in control, he who must be affirmed in the truth of his own nature, and allowed to do what he wills, whereas I, his servant, affirm nothing of myself apart from him, but am totally at his service, entirely subject to him, wholly his 'minister'.
— Congar, "St. Thomas: Servant of the Truth"