Paul, an apostle of Christ Jesus by the will of God, To the saints who are in Ephesus, and are faithful in Christ Jesus.
Ephesians 1:1 ESV
Paul often addresses Christians as saints in all his letters, but the theology of Ephesians is especially relevant to why he does so. Saint means holy one. It is related to the verb “sanctify,” make holy.
According to Acts, Paul claimed his calling as an Apostle came with a specific target group:
And I said, “Who are you, Lord?” And the Lord said, “I am Jesus whom you are persecuting. But rise and stand upon your feet, for I have appeared to you for this purpose, to appoint you as a servant and witness to the things in which you have seen me and to those in which I will appear to you, delivering you from your people and from the Gentiles—to whom I am sending you to open their eyes, so that they may turn from darkness to light and from the power of Satan to God, that they may receive forgiveness of sins and a place among those who are sanctified by faith in me”
Acts 26:15–18 ESV
Paul’s calling as an apostle was directly tied to a mission to make Gentiles saints and to authoritatively declare them to be saints. Thus, the most relevant precedent for calling them holy might be found in Deuteronomy 7:6. “For you are a people holy to the LORD your God. The LORD your God has chosen you to be a people for his treasured possession, out of all the peoples who are on the face of the earth.” That holiness no longer separates Israel from the nations or Gentiles. Now it includes both Jews and Gentiles who are “in Christ.”
That new unity is exactly what Paul spells out in Ephesians (2:11-22) and also claims to know by revelation given to him as part of his Apostolic calling (3:1-13).