The Angelic Doctor Against the Errors of the Greeks - Chapter 3

 How the Holy Spirit is to be understood as third light.

Even more calumnious seems the inference to be drawn from a text of the Cypriot bishop St. Epiphanius: “The Holy Spirit is the spirit of truth, a light third in order from Father and Son.”  Now where there is unity, there is no order of first and third. But Father, Son, and Holy Spirit are one light,  just as they are one God. Therefore, just as Catholics cannot say that the Holy Spirit is a third God distinct from Father and Son, so they cannot say that he is as third light.

He is said, however, to be the third person because of the plurality of persons. From the fact, then, that he speaks of a third light, it follows that there are three lights. This Ephiphanius expressly states in a subsequent passage: “All other things are called lights by reason of position or composition or appellation, not however being similar to these three lights.” 

On the other hand, it might be said in explanation that a light implies a certain origin; for light  is what is diffused from some source and of itself can also diffuse further light. In this way the word light can be stretched to connote personal properties in virtue of the diffusive property of light, even though light by nature properly pertains to the realm of essence. Noting this the said Father spoke of a third light and three lights in God. But this statement should not be pressed too far. Rather, Father, Son and Holy Spirit should simply be confessed as one light.