The apostle Paul considered everyone who is baptized into the body of Christ a saint.
The Roman catholic church has a rigorous process by which a person is elevated to sainthood. First s/he is declared a "servant of God," then after further examination determined to be "venerable or heroic in nature," followed by more scrutiny and declaration of "blessed," and finally, canonization as saint.
The author Jim Finlay says of a saint, "In them God finds no obstacle."
Someone unknown to me described a saint as "a stained glass window through which God's love shines."