Christ is all-knowing, and wished to inform his beloved disciples about the secrets of the heavens and the future, but the abilities of soul and mind could not absorb such truths in full. So we cannot logically realize that Christ is seated at God’s right hand in heaven, yet at the same time dwells in our hearts, unless the reasoning of the Spirit would illumine us. Similarly, we cannot naturally grasp that God is one in three persons. This the human brain is unable to comprehend, but the Spirit helps our infirmities and illumines our minds. He can reveal the secrets of the future to us, and the hidden thoughts of hearts, for he knows the profundities of the Holy Trinity.
Christ foretold that the Spirit of truth would come and lead them into all truth. What is truth? Jesus did not employ the plural "truths" as if describing earthly facts, but in the singular, as when he said, "I am the Truth". The revelation concerning the Spirit’s coming means that he will lead us into the fullness of Christ in action and essence. In so far, Jesus is not a mere man, but the Father is in him, and he in the Father. So the guiding into all the truth means knowledge of the Father and our abiding in His love and survival in eternity. The word "Truth" in the gospel does not mean legal truth or logical validity or even moral truthfulness only, but its meanings are wider, and cover all facts, general and specific. Thus the Spirit leads us to heavenly truths that we may know God in Trinity and experience His miraculous powers.
With all that, the Holy Spirit is an independent Person, speaking, listening, with a free will, yet at the same time he does not do anything save the Father’s will. He does not come with special thoughts, but tells us what the Father has said. In the Holy Trinity there is nothing except mutual submission in the freedom of love. He is faithful in the witness he transmits from the Son of God. Thus he wishes to build up the whole Church as Christ’s body, that she might be perfect at the coming of Christ, her Bridegroom.