I don't know about you, but I feel like the Holy Spirit is the least understood aspect of the Trinity. I've been doing some reading and here are some quotes that I came across that helped me out:
"The testimony of the Spirit is more excellent than all reason. For as God alone is a fit witness of himself in his Word, so also the Word will not find acceptance in men's hearts before it is sealed by the inward testimony of the Spirit. The same Spirit therefore who has spoken through the mouths of the prophets must penetrate into our hearts to persuade us that they faithfully proclaimed what had been divinely commanded...because until he illumines their minds, they ever waver among many doubts."
-John Calvin "The Institutes"
(In reference to 1 John 5:6-11) "In other words, 'The testimony of God' that is, the inward witness of the Spirit, is greater than any human witness-including, I think the apostle would say in this context, the witness of our own judgement. And what is that testimony of God? It is not merely a word delivered to our judgement for reflection, for then our conviction would rely on our own reflection. What is it then? Verse 11 is the key, 'This is the testimony, that God gave us eternal life.' I take that to mean that God testifies to us of his reality and the reality of his Son and of the gospel by giving us life from the dead, so that we come alive to his self-authenticating glory in the gospel. In that instant we do not reason from premises to conclusions; rather we see that we are awake, and there is not even a prior human judgement about it to lean on. When Lazarus wakened in the tomb by the call or the 'testimony' of Christ, he knew without reasoning that he was alive and that this call awakened him."
-John Piper "God is the Gospel"
"Calvin affirms Scripture to be self-authenticating through the inner witness of the Holy Spirit. What is this 'inner witness'? Not a special quality of experience, nor a new, private revelation, nor an existential 'decision', but a work of enlightenment whereby, through the medium of verbal testimony, the blind eyes of the spirit are opened, and divine realities come to be recognized and embraced for what they are. This recognition is as immediate and unanalyzable as perceiving of a color, or a taste, by physical sense-and even about which no more can be said than that when appropriate stimuli were present it happened, and when it happened we know it happened."
-J.I. Packer "Calvin the Theologian"
This last quote I just loved, so I'm adding it:
"A pernicious error widely prevails that Scripture has only so much weight as is conceded to it by the consent of the church. As if the eternal and inviolable truth of God depended upon the decision of men! ...Yet, if this is so, what will happen to the miserable consciences seeking firm assurance of eternal life if all promises of it consist in and depend solely upon the judgment of men?"
-John Calvin "The Institutes"