The Holy Spirit, aka The Holy Ghost, is sometimes sort of the forgotten member of the Trinity. We’re on much more familiar terms with the Father and the Son, but the Holy Spirit? He’s sort of like the red-headed step child of the Godhead.

When we think about the Holy Spirit, we think in terms of mystery. Of the strange. Of the borderline paranormal. And it’s the Holy Spirit that turns Christians from regular people into raging crazies in the eyes of the world.

From the perspective of Scripture, however, you find the mysterious and the tangible, the logical and the incomprehensible, when you talk of the Spirit of God. What you don’t find, however, is the relegation of the Holy Spirit to a distant third in the pecking order of the Trinity. The Holy Spirit is the most vital part of the Christian experience; indeed our ongoing Christian life rests solely on His shoulders.

But given our seeming unfamiliarity with the Holy Spirit, how do we know if something we are experiencing is really the His work? How do we know some urging or feeling is really from the Holy Spirit, or just last night’s pizza that went down badly? You might say that it’s “just a feeling” that we have when the Spirit is at work; the problem with that is that our feelings are unreliable at best; they are liars at worst. No, there is a better way to know that the Spirit has been at work – one rooted in the most famous and historical example of the Spirit’s work we have recorded.

If we turn to Acts 2, we find what was happening in the house spilled into the streets.

Inside the house as the early disciples of Jesus were gathered together, waiting for something to happen as Jesus had promised them, the Holy Spirit fell. With the sound of a mighty wind and the visual demonstration of power of flaming tongues resting on each disciple, the disciples tumbled out into the crowd at Pentecost to declare the wonders of God. At first, they were met with great confusion. Maybe some laughter. Probably some cynicism. But the crowd was prompted to collectively ask a singular question:

And all were amazed and perplexed, saying to one another, “What does this mean?” (Acts 2:12).

Peter rose to the occasion. In a stirring sermon, he connected the news of the death and resurrection of Jesus to the prophecies of old. Powered by the Spirit, the preaching of God’s Word did not return void, as the crowd there came to the sudden realization that they were in desperate need of forgiveness. And so the question of the crowd changed:

When they heard this, they came under deep conviction and said to Peter and the rest of the apostles: “Brothers, what must we do?” (Acts 2:37).

That difference is key. The first question was a matter of mere curiosity: “What does this mean?” But then the question changed to one of volition: “What must we do?” And here we find one of the tell tale ways in which we can identify the work of the Holy Spirit.

Namely, that the work of the Holy Spirit produces transformation. This is, overall, the primary work the Holy Spirit does in our lives:

“Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. We all, with unveiled faces, are looking as in a mirror at the glory of the Lord and are being transformed into the same image from glory to glory; this is from the Lord who is the Spirit” (2 Cor. 3:17-18).

This is still one measuring stick we can lay beside what we see in our lives and the world around us. Are we and others being moved toward holiness? Toward following Jesus more fully? Toward a heart aligned with God’s purposes? Is our curiosity being moved to conviction, and our information being moved to transformation? If so, and if all those movements are aligned with God’s revealed will in the Bible, then we can be confident that what we are experiencing is the work of God’s Spirit just as the early church was.

The early church moved forward thanks to the unmistakable work of the Spirit of God in the lives of both the old and new disciples of Jesus. What we find here is not just a historical accounting of the birth of the Christian movement; we find here a lasting testimony of how dramatically and quickly the Holy Spirit can change a person to their very core. This same Spirit that worked so dramatically at Pentecost is alive and well and working today.