The doctrine of the Trinity is no doubt one of the sacred mysteries of the faith, a wonder before which we bow and adore. At the same time, the statements that most Christians even in sound orthodox churches hear about the Trinity (one God, three Persons, Father, Son and Holy Spirit) while most certainly true and necessary, are not everything that can be said about this doctrine. Consider this:
[Riissen (IV,14)]: “What the difference is between generation of the Son and the procession of the H. Spirit cannot be explained and it is safer not to know than to enquire into it. The Scholastics would look for the difference in the operation of intellectus and voluntas, so that the generation of the Son is brought about by means of intellectus, whence he is called the wisdom of God; but procession by means of voluntas, whence it is called love and charity. But as this is said without Scripture, it involves rather than explains matters. Those talk more sanely, who babbling in such a difficult matter find the distinction in three things. (1) In principle: because the Son emanates from the Father alone, but the H. Spirit from Father and Son at once. (2) In mode: because the Son emanates per vim generationis, which culminates not only in personality but also in likeness, on account of which the Son is called the image of the Father and according to which the Son receives the property of communicating the same essence to another person. But the Spirit does so by spiratio, which ends only in personality, and through which the person who proceeds does not receive the property of communicating that essence to another. (3) In order: because as the Son is the second person, but the H. Spirit the third, generation by our way of thinking, precedes spiratio, although really they are co-eternal.
From Heinrich Heppe, Reformed Dogmatics