Source: Trinity, Robert Morey, pp. 73ff
The Trinitarian begins with the a priori assumption that the triune God of Father, Son and Holy Spirit will be incomprehensible because God is essentially incomprehensible in His nature. (This means that God cannot be fully understood; not that he cannot be partially understood. We can only understand God partially.) The inescapable truth is that God will always be greater than our finite capacity to understand fully or to explain exhaustively.
Our failure to understand or explain fully the Trinity or any other aspect of God is not due to some defect in God or in His revelation. The "defect," if it can be called that, is nothing more or less that the reality of our own finiteness. Man is a finite creature with limited abilities intellectually as well as physically. Indeed, we do not even have an exhaustive knowledge of ourselves much less God!
If the Trinity were fully understandable, this would be an indication that it is erroneous. But the Trinity is so deep and so high a concept that we expect to be left with unresolved problems and unanswered questions. (1Co 1:31)
Any god we could fully understand and explain would be less than what we are. He who can understand God would be greater than God.
The incomprehensibility of God is, thus, not a surprise to the Trinitarian. If there is an infinite God who has revealed Himself to finite man, then it only follows that what is revealed will go beyond man's finite capacity to understand.
Trinitarians do not mean to imply that God is irrational or illogical when they speak of His incomprehensibility. They simply mean that God is beyond man's capacity to understand or explain exhaustively. In this sense, God is beyond human reason without being contradictory of it.
If we begin with the a priori assumption that, if something cannot be fully explained, it must be rejected, then we must reject the love and peace of Christ because they are "beyond our comprehension" Eph 3:19; Php 4:7.
Job 5:8-9; 11:7-9; 36:26; 37:5; 42:1-4,6; Psa 77:19; 92:5; 139:6; 145:3; Ecc 3:11; 8:17; 11:5; Isa 40:28; Rom 11:33-34;
Eph 3:8,19; Php 4:7