The group found at the beginning of Acts 19 were stuck in the time, having received only the teaching of John the Baptist and not being familiar with the finished work of Jesus Christ in all its fullness. Upon the completion of God’s revelation, the NT knows of no such person as a Christian without the Spirit. It is impossible for the Jewish Scriptures or the teachings of John the Baptist to have gone to places that the gospel of Jesus Christ did not follow at some point. And the NT is very clear about the Holy Spirit indwelling all true believers—Rom. 8:9…anyone who does not have the Spirit of Christ does not belong to him.
But how do we know who has the spirit? That’s answered in Rom. 8 a few verses later in vv.12-13—those who have the Spirit don’t live according to the flesh and instead put to death the deeds of the flesh. Obviously, this has nothing to do with being churched or going on mission trips or giving money to Christian organizations. Unbelievers can do those things, but they cannot put to death the flesh for they still are in bondage to it.
Related to that is what Paul writes elsewhere in Col. 1:9…we have not ceased to pray for you, asking that you may be filled with the knowledge of his will in all spiritual wisdom and understanding, 10 so as to walk in a manner worthy of the Lord, fully pleasing to him: bearing fruit in every good work and increasing in the knowledge of God;
That’s also what the Spirit will produce in a believer, for a true believer will never be satisfied with his or her level of spiritual understanding and knowledge of God. The Spirit will give the believer that great ambition as set forth by Paul in 2 Cor. 5—to please Christ above all and that won’t happen apart from growing in the truth. A growing Christian is one who is developing more and more accuracy in the Scriptures. God has declared to be very honored by that, so let’s pursue those things at a cost to other things, instead of pursuing other things at a cost to our accuracy and growth.