Paul, an apostle of Christ Jesus by the will of God, to the saints who are in Ephesus, and are faithful in Christ Jesus: Grace to you and peace from God our Father and the Lord Jesus Christ.

and are faithful in Christ Jesus

“Christ” means essentially King in Paul’s letters, with association with certain promises that a king would come and save. It would be more natural for us to read Paul write of those who are faithful to Christ Jesus, but that is not what he says. Rather, Jesus is not only the one to whom we are loyal, but He is the context in whom we walk the walk. Jesus is not only the king, but he is, in some sense, the realm or land in which our behavior can make any sense.

This idea actually goes back to David who is treated like he is somehow the land in which the Israelites live:

Then all the men of Israel came to the king and said to the king, “Why have our brothers the men of Judah stolen you away and brought the king and his household over the Jordan, and all David’s men with him?” All the men of Judah answered the men of Israel, “Because the king is our close relative. Why then are you angry over this matter? Have we eaten at all at the king’s expense? Or has he given us any gift?” And the men of Israel answered the men of Judah, “We have ten shares in the king, and in David also we have more than you. Why then did you despise us? Were we not the first to speak of bringing back our king?” But the words of the men of Judah were fiercer than the words of the men of Israel (2 Samuel 19.41-43).

And then again in the very next passage (ch. 20; emphasis added):

Now there happened to be there a worthless man, whose name was Sheba, the son of Bichri, a Benjaminite. And he blew the trumpet and said,

We have no portion in David,
and we have no inheritance in the son of Jesse;
every man to his tents, O Israel!

Christians are “in Christ” just as Israelites were “in Israel.” Christ is the environment in which they live and act.

In this context, we should mention that the faithfulness that Paul attributes to believers is not in any way contrary to the Bible’s message of grace and being right with God only through faith rather than earning such a position through moral behavior. Rather, in Christ, they are continually forgiven so that their obedient deeds, no matter how imperfect or wrong, are gladly received by God as a loving Father. As believers who have entrusted themselves to Christ alone, they have been faithful to the call of the Gospel and our counted faithful as they work that faith out in their lives, despite their shortcomings.

(This, by the way, has tremendous implications for how believers should treat others for whom they are responsible and with whom they are related. Paul will spell this out.)