As well as Chalcedonion Christology (cf. Part II), the Trinity plays a central role in Anselm’s exposition of the Atonement. In fact without the Trinity Anselm’s understanding of the Atonement will be rendered invalid. Anselm is a passionate advocated of the Triune God, be it his understanding of soteriology, creation or anthropology, they are all governed by his appreciation of the Trinity.
In Cur Deus Homo, Anselm reiterates the classic view that the work of one of the divine persons belongs to the three Trinitarian persons. Due to the Turin nature of God, through the Incarnation, the divine Son, being different from the Father, offers himself to the self-same divinity of the Father and the Holy Spirit, hence the offering he makes is an offering made to his own honour – the three persons. In fact, with out plurality in oneness Anselm’s soteriology will simply collapse. A singular monad, precisely because it/he is singular, cannot offer to it/himself the necessary offering discussed in Part I. However, since God is Trinity, plurality of persons in unity and oneness of nature, the Son can make an offering to the Father. The one divine nature that the Son shares with the Father and the Spirit is the factor value of the ransom required by God; hence the divinity of the Son is the perfect match for the magnitude of the recompose required by God of humankind. 
To Anselm the particularity of the Son determines the unique nature and character of the content of salvation. Anselm pays a particular attention to the particularity of the Son as the fitting person who should assume human nature; the Incarnation was uniquely suited to be the mission of the Son, rather than the Father or the Spirit. For if the Spirit was to be incarnate, Anselm argues, then there were going to be two sons, one human the other divine. Likewise, if the Father was to be incarnate, as well as two sons, there were going to be two grandsons. I.e. the Father would have been the grandson of the parents of Mary, and the Word would become the grandson of Mary, for he will be the son of here son.This argument may seem strange to 21st century ears, but as bizarre as it is, it does reveals two important features of the importance of the Trinity in Anselm’s soteriological thought:
Firstly, it emphasises the importance of the concept of union. I.e. in salvation, through Christ, humanity is brought into union with God, to share in the divine life, which consequently re-establishes human life. At times, Anselm is criticised for not taking into account the key patristic notion of theosis.  The criticism to a certain extent is justified, for Anselm is very much at home with the Augustinian concept of Original Sin resulting from the Fall, hence his soteriology is more of a looking back with a focus on reparation of the human condition and restoration to per-fall condition. However, because of the Incarnation, as well as mainly looking back, Anselm is also looking forward. For, due to the Son’s assumption of human nature, the man Jesus has a permanent place in the life of the Trinity. The existence of the new man on the heavenly throne secures a future for humanity, as Deme argues, for Anselm, ‘The incarnation is the proof of the existence of redeemed humanity, and therefore the proof of man’s existence’.As Anselm, when considering the Cross, puts it ‘in the Incarnation of God…no humiliation of God came about: rather…the human nature was exulted.’ Here, though resurrection and theosis is not explicitly mentioned, nevertheless, implicitly, Anselm’s belief amounts to similar outcome.
Secondly, and this is important for modern Trinitarian discussions, Anselm takes the Trinitarian titles, Father, Son and Spirit, with utmost seriousness. The names, Father, Son, and Holy Spirit are not mere empty symbolic titles, a simple means of communication with no real substance to them. To Anselm, the names are what they suggest, the persons are truly and eternally Father, Son and Spirit. Anselm is not threaten by a God who is Father, Son and Spirit; the concrete identity of the Turin persons does not pose andy threat to the value and identity of human beings. As we saw above, when thinking of the Incarnation, it is precisely the Son’s Son-ness that renders him suitable to assume human nature. It is because he is the Son of God by nature he can give us to be sons/children by adoption. this means that the content of salvation entails more than forgiveness, to include the gift of being indwelled by the Son’s Spirit and to be given the privilege of calling his Abba our Abba. This being the case, we can conclude that without God’s Truin identity Anselm’s soteriology falls apart. To be continued…
 As early as second century it was understood that the Father works through the Son by the Spirit, cf. St. Irenaeus of Lyons, On Apostolic Preaching, I. 5, the same is argues by Augustine in fifth century cf. Augustine, On the Trinity, XIII 4.
 Cf. CDH. II. 18.
 Cf. Deme Daniel, The Christology of Anselm of Canterbury, pp.104-105.
 Cf. CDH. II. 9.
 Cf. O’Collins, SJ Gerald, Jesus our Redeemer: A Christian approach to Salvation, p.134-140.
 Cf. Deme, ibid, p.182.
 CDH. II. 7.