What is being given to the saints?

The word call is not present in the Greek. The NASB provides a more literal translation: “Here is the perseverance of the saints.” The addition of the word “call” into the translation changes the sense of the passage from a gospel message into an obligation, as if John were summoning the saints to make a point of enduring in the face of tribulations. (Of course, in other places Scripture certainly makes clear that saints have an obligation—call—to steadfastness, even when the going is rough.) The Holy Spirit in this verse gives a gospel reality, an encouraging promise: given the inability of any devilish force to prevent the proclamation of heaven’s message all over the earth, saints may be confident and optimistic about the/their future.