I will briefly show you the logic of this movement of modern philosophy towards Trinitarianism. The advance movement of thought in this century, far from expelling the Trinity from its place in the mind of Christendom, has caused it to strike deeper root and grow with fresh vigor.
This is the argument in a nutshell: Love, the basis of all God's moral attributes, cannot exist in that simplicity, that abstract self identity of the divine nature which is the essence of the Unitarian conception. Why? If God existed from eternity before He created a person on whom His love could pour itself, He was from eternity having the active, diffusive principle of love in His bosom, the very substance and substratum of His being, with no object to love. This is unthinkable! Eternal love without a personal object! Hence philosophy must either deny the existence of love in God, together with all its manifestations in the forms of holiness, justice, wisdom and truth, or supply such an object from eternity. This first alternative divests God of all His moral attributes and takes us more than half way to atheism. The first verse of St. John's Gospel supplies an object of the Father's love before His first creative act: "In the beginning (from eternity) was (not was created) the Logos, and the Logos was with God (face to face is the idea in the Greek), and the Logos was God." This Logos became flesh, and while tabernacling in humanity had a memory which went back beyond the creation of the universe to "the glory which He had with the Father before the world was." In these words of unsurpassed majesty, which no creature ever dared to use, we have an enlargement and enrichment of the concept of Deity in striking contrast with what Phillips Brooks called "the meagre God of Unitarianism," a simple, abstract unity, a mere cause or primal force. Hence the necessity of at least a dualism of persons in the divine nature to sustain the completeness of the divine life and character.
The New Testament reveals such a dualism and adds the Third Person as essential to the perfection of full-orbed divinity. Hence we see that the Trinity is not a doctrine which has been arbitrarily imposed upon faith by external authority overriding reason, but it is one which accords with reason, after it is revealed, and explains and supports Christian experience. Every evangelical believer who through faith in Christ by the illumination and impulse of the Holy Spirit has had conscious access to the Father resulting in forgiveness and communion, has tested the doctrine of the Trinity and found it true. Its deniers must reckon with the best philosophy representing the demands of the highest intelligence; then they must convince of their stupendous delusion the millions who have through faith in the divine Christ experienced the witness of the Spirit attesting their adoption and assuring them of forgiveness. But every growing Christian verifies the truth of the Trinity every day of his life. He comes to the Father through the mediation of the Son and receives the Holy Spirit as the comforter, helper, guide, light, life and wellspring of joy, just as every astronomer proves the truth of the Copernican theory of the solar system by using it and arriving at results experimentally verified by the use of his telescope, "Through him (Christ) we (Jews and Gentiles) both have our access in one Spirit unto the Father" ( Eph. ii. 18).
— The Gospel of the Comforter, Chapter 2.