These things saith the Amen - That is, He who is true or faithful; from אמן aman, he was tree; immediately interpreted, The faithful and true witness. See Rev 1:5.
The beginning of the creation of God - That is, the head and governor of all creatures: the king of the creation. See on Col 1:15 (note). By his titles, here, he prepares them for the humiliating and awful truths which he was about to declare, and the authority on which the declaration was founded.