Does believing in Christ activate God's word or just make us think and act different?

Since I don't know the context it is hard to explain what your pastor meant exactly. I will try to give a general answer to the question about the Spirit as a strengthening and guiding Helper in our lives.

(This answer will ignore the teaching in the Bible about the Spirit as co-creator with Christ and the Father, the Spirit as the endowing power that enabled the earthly Jesus to perform miracles and indeed even submit to the Father's will in sacrificing himself, etc.)

Two points before I really get started.

First of all, the Bible never intends to solve the paradox between our responsibility as human beings and our dependence on God. It's like the bike having two pedals. You will need both to push it forward.

Second, the Spirit will guide us to maturity. Being mature means not having to be told every minute detail that you're supposed to do. Such knowledge will be internalized when you're grown up.

Occasionally, though, there may be moments when specific guidance is needed, especially for ministry. But let's start at the beginning.

1. The Spirit is not completely unknown to any living human being. "He himself gives everyone life and breath and everything else." Acts 17:25
2. God can, through the Spirit, speak to (or prompt) anyone or speak through anyone. Like Cyrus of Persia or through Balaams donkey. The person may or may not be aware that it is God speaking.
3. It was through the Spirit of God that saving faith is bestowed to us. While Christian traditions may interpret the how of this differently, the fact that the Spirit is a key part of this is unarguably true.
4. It is through the Spirit that you're born again. The phrase may have been overused or even abused, the Biblical truth never becomes reduced to nothing. Being born again means a fundamental inner transformation. It makes you partake of the nature of Christ himself. But our sinful nature is not erased. As of us being born again, the Spirit takes begins to dwell in us. The verb dwelling is never applied to unbelievers.
5. The Spirit will illuminate the Word of God, making it more than a an objective truth, but also a personal communication from God.
6. The Spirit will guide us and strengthen us by giving (or withdrawing) inner peace. That peace needs us dialing down and taking time to communicate with God through individual and communal prayer, worship and listening to Gods Word. It can be silenced by stress, fear and doubt.

The greatest working of the Spirit and the most awesome effect of the Spirit dwelling in our beings, is the release of the cry Abba! Father! The Spirit makes our being daughters and sons of God more than a judicial state, it enables intimacy in our communion with the Father. (Rom 8:14-17; Gal 4:6)

So far I think every believing reader on this forum will agree. I will provide a few more points, where admittedly Christian traditions differ, but this is how I interpret the Bible:

1. The Spirit can "fill us", or "come upon us". This happened to Peter at least 3 times in Acts (2:4; 4:8; 4:31 - exact same verb in the Greek original). Through this we get power for ministry in that situation.
2. The Spirit will endow us with Spiritual gifts, through which we can build each others up; Through them we can exhort, inspire, comfort, teach, heal, etc.
3. We can be "filled" with Spirit. The NT has other verbs that speak of a more continual state. But there is no exact distinction between points 5-7.
4. We can be guided through the Spirit, not as in being taught doctrine apart from the Bible, but as in being helped in our individual service for God.
5. The Spirit will help us pray, through inner promptings, prophetic inspiration or the gift of tongues.

Finally (at least in this answer), building this primarily on Gal 5:13-26, but also many other sources.

1. The Spirit will empower the godly life, helping us resist evil and temptation and provide strength to avoid sin.
2. Through the Spirit "the fruit (singularis) of the Spirit", primarily love, faith and hope, but in essence encompassing every good and Christ lie personality trait. Thus, God is working so that I both in will and in deed do the will of God. My will is not reduced from the equation, but it is affected by God. He will make me want what is good.

The Christian life may contain many periods of great enthusiasm, personal revival. I personally think of these as the blossoming season in the spring. Fruit, however, is seen during the dull autumn. Fruit grow slowly. (While gifts are given instantaneously.)

So in essence - yes the Spirit of God is in a sense an external force. God is God and I am me. But the Spirit of God dwells within me, since I have been made a child of God thanks to Christ. Thus, the Spirit is working within me, but also through other people in my vicinity.