Several characters' names are different in the novelization:
- Robert Thorn's name is given as Jeremy Thorn;
- Holly's name is given as Chessa;
- Keith Jennings's name is given as Haber Jennings;
- Father Brennan's name is given as Father Edgardo Emilio Tassone;
- Father Spiletto's name is given as Father Spilletto;
- Maria Sciana's name is given as Maria Avedici Santora;
- Mrs. Baylock's real name is given as Sister Maria Teresa, also known by her Satanic name B'aalock; and
- The Hellhound's name is given as Shepherd.
Some minor details differ as well; the Episcopalian wedding of the film is said to be a Catholic wedding in the novelization (there is no Episcopal Church in England, where the national Church of the Anglican Communion is quite naturally the Church of England). The Etruscan cemetery at Cerveteri is named the Cimitero di Sant'Angelo, which is said to include "the Shrine of Techulca, the Etruscan Devil-God."
Additional details are offered, such as the names of the hospital where Damien was substituted for the Thorns' natural child (the Ospedale Generale), the house where the Thorns reside in England (Pereford House), and Kathy's psychiatrist (Dr. Charles Greer). Robert's professional background is more detailed, including his standing as an industrialist and the President's economic adviser and chair of the World Economy Conference (which had moved from Zurich to Rome). Additional material is offered on the backgrounds of the Thorns, Jennings, Father Tassone, and Mrs. Baylock (who is significantly more rotund than in the film).
Notably, Tassone is revealed to be a Portuguese missionary who was recruited to the Satanist cult by Spilletto (thus explaining his "666" birthmark). Spilletto's coven at Rome undertakes covert terrorist and subversive activity to foment violence and instability throughout the world, including a bombing campaign by two Irish nuns (B'aalock and B'aalam) to stir up religious hatred between Catholics and Protestants in Ireland. Tassone goes on to carry out a covert operation in Southeast Asia in 1968, sabotaging a ceasefire between North and South Vietnam, as part of a plot to spread Communism throughout Cambodia, Laos, Vietnam, Thailand, and the Philippines. He subsequently helped Idi Amin seize power in Uganda, and facilitated his takeover of the Caucus of African Nations. Tassone, Spilletto, and Sister Maria Teresa (i.e., B'aalock) carry out the plot to incarnate the Antichrist; Tassone murders the Thorns' son in Rome, and B'aalock oversees the impregnation and delivery. Tassone is subsequently haunted by dreams of Christ, pushing him to seek redemption by helping Thorn destroy Damien.
Additionally, it is revealed that Damien is the third attempt to incarnate the Antichrist. In 1092 and 1710, previous attempts were foiled by members of the Bugenhagen line, "religious zealots, the watchdogs of Christ" who killed or rendered harmless the previous Antichrists. (This same information is repeated by Damien himself in the novelization of The Final Conflict, after Brother Benito's failed assassination attempt.)