In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory. – Ephesians 1:13-14
This passage at first seems like an ordinary discussion of conversion. But within the context of the previous passage, it becomes clear that this is specifically speaking about Gentile conversions. The specific references to the gospel, truth and the sealing of the Holy Spirit fit well with the testimony of Luke in the book of Acts (Acts 8:14-17; 10:34-48; 11:15-18).
Shortly after affirming the special and declarative nature of the Holy Spirit upon the Gentiles, Paul switches back to the informative “we.” The Holy Spirit is a testimony to both Jews and Gentiles. He is the promise to a final inheritance to the Jew first and also to the Greek.