The original question of this thread is, “Is modalism logically wrong?” It should be noted that theology does use logic as an aid to achieving the end of theology (and, thus, any position that contains a logical contradiction must be rejected by the theologian). However, the primary task of theology is not merely to consider what is logically possible; but, rather, the task of theology is to consider more deeply, and explain more precisely, what God has revealed about Himself (through the Scriptures and Tradition of the Church, as these are understood by the Magisterium of the Church).
This being said,* even if *one were to hold that the modalist doctrine does not contain a logical contradiction, it still must be maintained that the doctrine does contradict what God has revealed about Himself (for, God has revealed that He is Father, Son, and Spirit, Each of Whom is distinct from the Others, and Each of Whom is the one God). For this reason, modalism is to be rejected as erroneous.
On the other hand, the Trinitarian doctrine gives an explanation which integrates the various truths that God has revealed about Himself; and it does so without contradicting reason. True, the Trinitarian doctrine goes above reason-- for reason could never have known of the Trinity, with certitude, unless God revealed it to be true (whereas, for example, the existence of God, and of His primary attributes, can be known, with certitude, by reason alone). But the doctrine does not contradict reason (and, in fact, given the fact that God has revealed Himself to be a Trinity, reason can begin to see that this doctrine is very reasonable).
*“Glory be to the Father, and to the Son, and to the Holy Spirit, as it was in the beginning, is now, and always, and unto the ages of ages. Amen.”
*
In Christ, The Second Person of the Blessed Trinity,
BrotherAlan
“Jesus Christ is Lord!” (Phil. 2:11)