How does the Spirit perform his ministry? The most obvious thing in scripture are these "gifts" of the Spirit.
"But one and the same Spirit works all these things, distributing to each one individually just as He wills" (1 Cor 12:11).
Part of the Spirit's role was to distribute various gifts "just as He wills." The scriptures gives a list of some of these gifts (1 Cor 12:1-11; Rom 12:6-8). It is important to note that the Spirit did not give the same gift to everyone, not every gift was miraculous, and having the Spirit was not a guarantee that someone might have a miraculous ability. Many of these gifts were related to teaching, communication, and edification, which were an absolute necessity when there was not yet a completed New Testament. The goal of these gifts was for the "common good" (1 Cor 12:7), and to grow as the body in Christ in love (Eph 4:16), which is the fulfillment of the law (Rom 13:8; James 2:8) and a requirement for ministry (1 Cor 13:1-3).
The Spirit could have chosen to continue to operate in the same way, but as time went on, the miraculous gifts waned and all but disappeared from the scene at the end of the first century. Paul mentioned that this would happen when the "perfect" or "complete" comes (1 Cor 13:8-10). Although he doesn't specifically identify what this "complete" thing is, it seems to be the complete revelation of Christ in writing, since the cessation of the miraculous gifts and the completion of the New Testament happened at the same time.
So, does this mean that the Spirit no longer active? Hardly. Scripture indicates that the Spirit has an active ministry, part of which directly involves the word and us. There is a close connection between the written word of God and the Spirit. It is "inspired by God" (2 Tim 3:16), or literally "God-Spirited" or "God-breathed." The scripture did not originate from man, but men who were "moved by the Spirit" spoke from God (1 Pet 1:21). So, it appears that the written word of God is part of the set of tools the Spirit has chosen to conduct his ministry. This is why the word of God is the "sword of the Spirit" (Eph 6:17) that we can use in our spiritual struggle against the flesh and the philosophies of the world. The word of God is "living and active" and "sharper than any two edged sword" and it can pierce as deep as our "soul and spirit" (Heb 4:12). It penetrates to our inner self and is God's surgical tool to sculpt, shape, and transform the "thoughts and intentions of the heart."
There is nearly a mystical quality about God's written word. It is not the same as reading Shakespeare or Homer. Part of growing "spiritually" and being "spiritual" has to involve lots of regular time with God's word. Since it is God's breath, so to speak, then we need to breathe regularly in order to have life. His word is life (Jn 6:63). We need to nor merely study it, but let it study us. It is God's two edged sword to probe our innermost being and sculpt us.