Thomas's trinitarian doctrine is commonly criticized as being abstract and unbiblical; several writers have offered defences against this charge, but these perhaps ignore too much the genuinely reticent and apophatic aspects of Thomas's thought. In three particular places, it is argued, Thomas can be read as deliberately saying things that are unexplained and unexplainable. The areas considered are: the notion of processions in God; the presentation of the persons as subsistent relations; and the relationship between the subsistent relations and the divine essence.
Kilby, K. (2005). Aquinas, the Trinity and the Limits of Understanding. International Journal of Systematic Theology, 7(4), 414-427. https://doi.org/10.1111/j.1468-2400.2005.00175.x