The beginning of the gospel. A Hebraism. No Article. Compare Hosea 1:2, "[The] beginning of the word of Jehovah by Hosea". It is the beginning, not of the book, but of the facts of the good news. See note on Mark 8:11.
gospel. glad tidings. See note on Matthew (Title).
Jesus Christ. See App-98.
the Son of God. See App-98.