In the twenty-fourth objection to the fifth article of the eleventh question of the Disputed Questions on the Power of God ‘Are There Several Persons in God?’ St Thomas suggests that,

the fullness of joy requires the companionship of several in the divine nature, because there is no pleasure in possessing a thing unless we share it with a companion, according to Boethius. Moreover perfect love is to love another as oneself.

No one seems to have found where it is that Boethius says this, but St Thomas seems to have been convinced that he did because he attributes the same argument to St Severinus in the Commentary on the Sentences 1.50.4 ad 4 where he applies it to the question of a plurality of angels in one species. In De Potentia he provides a counter argument against the plurality of persons in God as part of the same objection.

to depend on another for the fullness of one’s joy and love is an indication of insufficient goodness in oneself. Hence the Philosopher says (Ethic. ix, 4) that the wicked through finding no pleasure in their own company seek the companionship of others: whereas the good seek to commune with themselves through finding pleasure in so doing. Now the divine nature cannot lack a sufficiency of goodness. Wherefore since one supposit of the divine nature has in himself all fullness of joy and love, there is no need to put several supposits, or persons in God.

The essence of St Thomas’s objection to this argument is contained in the body of the article “we must attribute to God every perfection that is in creatures, as regards the essence of the perfection absolutely but not as regards the way in which it is in this or that one.” So far as reason can discover the need of man to befriend his neighbour is merely a reflection of his insufficiency.  Friendship, reciprocally willing the good of the other for the other’s own sake, is not, according to St Thomas, a pure perfection. It is a perfection for creatures on account of their limitations. Except, St Thomas is not precisely committed to this, he may just hold that friendship cannot be known to be a pure perfection by natural reason. Could it not be that the doctrine of the Trinity reveals to us that friendship is a pure perfection but that we could never know this unless we are the recipients of the Triune God’s self revelation? The argument St Thomas considers here is most famously associated with Book 3 of Richard of St Victor’s De Trinitate. Richard argues that, being supremely good, God must be supremely loving and perfect love requires the love of the other and perfect love is unselfish. Thus God  must have coequal consubstantial second person to love from all eternity and these must share their love with a third person also coequal and consubstantial. Some people (not Richard) try to rescue Richard’s argument from the charge that it seeks to prove by natural reason the doctrine of the Trinity by saying that it pertains to the nature of charity not to natural friendship. If this were the case then the knowledge that friendship is a pure perfection would be indirectly derived from revelation rather than from reason.

It seems to me there may be something to this rescue attempt and it would explain the necessity of faith in the Trinity for salvation. It is clear that we need to accept as a gratuitous offer, God’s offer of friendship in order to accept that offer. It must be revealed to us and we must accept it qua revelation. It is also clear that after sin we need to accept as a gratuitous offer, God’s offer of redemption in order to accept that offer.It must be revealed to us and we must accept it qua revelation. But why do we need to know that friendship itself describes the inner life of God? Certainly we need to know that Jesus is God in order to believe in Him. It would be difficult to explain the Gospels unless we knew that there are two persons in God but that does not tell us why it is necessary for salvation to know this, or to know that God is three persons in one substance. But if the possession of charity inherently entails an implicit knowledge that God is three persons in one substance, then it is easier to understand why faith in Christ as a divine person necessarily renders explicit the implicit knowledge of the Trinity inherent in supernatural charity in any order of providence.

Of course, there is a form of knowledge directly dependant on charity, the connatural  knowledge of God consequent upon charity: wisdom. Everyone who is in a state of grace possesses the gift of wisdom. “Although wisdom is distinct from charity, it presupposes it, and for that very reason divides the children of perdition from the children of the kingdom.” This wisdom, I suggest, bestows upon all those who possess it the implicit knowledge of the mystery of the Holy Trinity, a knowledge which is necessarily rendered explicit by the explicit faith in Christ the Redeemer through which we appropriate the satisfaction He offered on the Cross. “The Uncreated Wisdom, which in the first place unites itself to us by the gift of charity … reveals to us the mysteries the knowledge of which is infused wisdom. Hence, the infused wisdom which is a gift, is not the cause but the effect of charity.”