The uncontroversial third article of the creed of 381 became highly controversial some centuries later and led to the Great Shcism between east and west. Where the creed had originally affirmed that the Spirit proceeded from the Father, the west, in an excess anti-Arian zeal, began to confess that the Spirit preceded from the Father and the Son (The Council of Toledo 589 AD). This addition however, was violently rejected by the Eastern Orthodox Churches. (Nicene Christianity, Seitz, 2001).
Many western Christians have no idea why it is that the eastern church gets so frazzled about the “Filioque” clause that was added to the Nicene Creed. The fact is however that for most western reformation Christians, the Trinity is not central to daily life. Yes, reformation Christians know that the Father is credited for bringing about the creation, the Son for redemption of that creation, and the Spirit for the giving life, but beyond that the doctrine looses its centrality. Not so for the first millennium churches. Each of the two communions, the Latin and the Greek, developed a different way of looking at the Trinity and therefore at looking at God.
The Trinity can only be approached from two perspectives, either from the oneness of the godhead (the west), or from the three-ness of the godhead (east). Since Augustine, the west saw the oneness as the place to begin, pure essence, thus in the godhead they concluded was a hierarchy. The Father is first, & He begets the Son, the Son is second, and from the two together proceeds the Spirit. The east however, saw no hierarchy in the godhead, only a circle of mutual self- emptying love. Hence, the Father begets the Son, and from the Father proceeds the Spirit. The Spirit proceeds to the Son & back to the Father, through the Spirit the Father begets the Son, who in turn gives himself to the Father. In this view God is a concilliar Godhead living in perfect wholeness and love, rather than a hierarchical godhead that flows from the top down.
These two differing views produce two different Gods, two different kinds of churches, and two different views of salvation. One sees a monarch that needs to be satisfied for the wrongs done to him, the other sees a co-equal God, each person serving the other. One sees salvation as keeping the hierarch happy by satisfying his legal requirements thru payment, the other sees salvation as on entering into the concilliar life of the godhead by the destruction of death. One sees the church as hierarchical, the lower serving the greater (papal), the other sees the church as concilliar, all serving one another with order.
Thus it is easy to see that unilaterally altering the creed was not only destructive to unity, but also injured the church, and salvation itself, and we in the west now bear its injuries.