Mark 1:15 - Ellicott's Commentary On The Whole Bible

The time is fulfilled. — The words are not found in the parallel passages of the other Gospels, and are interesting as embodying the same thought as St. Paul’s “in the fulness of time” (Galatians 4:4; Ephesians 1:10). So, too, St. Mark adds “believe the gospel” to the simple “repent” of St. Matthew, and gives “the kingdom of God” instead of “the kingdom of heaven.”