There is a possible reference to the Trinity in Acts 18. The passage under consideration is Acts 18:24-26:
Now a Jew named Apollos, a native of Alexandria, came to Ephesus. He was an eloquent man, competent in the Scriptures. He had been instructed in the way of the Lord. And being fervent in spirit, he spoke and taught accurately the things concerning Jesus, though he knew only the baptism of John. He began to speak boldly in the synagogue, but when Priscilla and Aquila heard him, they took him and explained to him the way of God more accurately.
The disputed phrase is the description of Apollos in verse 25, “And being fervent in spirit. . .” Is Apollos zealous in his own spirit, or was he zealous in the Spirit (i.e. the Holy Spirit)?
In his Catena on the Acts of the Apostles, Chrysostom takes it as a reference to the Holy Spirit. But this may be due to the established Patristic habit of talking all non-specific occurrences of “the spirit” as referring to the Holy Spirit. Origen details this in On First Principles 1.3.4-8. Even so, this view is also held by Calvin, Henry, and Marshall (though he only says it is “probable” that Luke is referring to the Holy Spirit).
All modern translations (that I could consult) take it as it is given in the ESV cited above—as a reference to Apollos’ own spirit. This seems to be the most common interpretation today. It is adopted by Bruce (NICNT), Munck (Anchor), and Stott (BST). (Although The Jerome Biblical Commentary and The Interpreter’s Bible (1954) both consider it a reference to the Holy Spirit.)
This aspect of uncertainty is not unique to Acts 18. There are several other passages in the New Testament that might be Trinitarian in nature, or might speak of only the Father and the Son. Passages that I know of like this include John 4:21-26; Colossians 1:9-10; 1 Tim. 3:14-16; 1 Peter 3:18; and Rev. 5:6. In the course of this blog-study these texts will be considered on an individual basis.
So for the sake of completeness, I mention Acts 18:24-26. Though I am not yet sure it should be mentioned for the sake of accuracy. Given what we have seen so far in the book of Acts, I am inclined to side with the translations and modern commentators. The text seems to emphasize two things for certain about Apollos: he was very able but not complete. If Apollos was “fervent in spirit” this makes sense of both facts. Given what Luke has shown so far about the work of the Trinity bringing full salvation, it is difficult to see how a man instructed in the way of Jesus and filled with the Holy Spirit could be missing essential truths about salvation.