It is easier for man to understand an angel bringing the Message than God speaking directly. As soon as one says, "God spoke," then a person says, "Has God lips to speak, has He then a form, is He then like a human being, is He then an entity, separate from our being?" And if these questions were answered, "Yes, it is so," that means that every possibility of a person coming to the understanding of God was covered by a pacifying answer. But if it is said, "An angel brought the Message," then people begin to think, "Yes, it is an entity who brought the Message." The postman could come from God to the earth and bring the Message. It's just like saying a dove brought a letter. One can quite understand that a dove could fly and bring a letter.