“The beginning of the gospel of Jesus the Christ, the son of the living God, according as it is said by the Holy Spirit, send an angel before his face, who shall prepare his way.”
(Gospel of the Twelwe Holy Apostles, syriac version)
Compare that incipit with Mark 1:1-3:
1 The beginning of the gospel of Jesus Christ, the Son of God;
2 As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
4 John did baptize in the wilderness
2 As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
4 John did baptize in the wilderness
Do you note the difference? In the first incipit the "angel" is Jesus. In the Mark's incipit the "angel" (angelos) is euhemerized as a human "messenger".
But then we remember about Exodus 23:20 et seq.:
“See, I am sending an angel ahead of you to guard you along the way and to bring you to the place I have prepared. 21 Pay attention to him and listen to what he says. Do not rebel against him; he will not forgive your rebellion, since my Name is in him. 22 If you listen carefully to what he says and do all that I say, I will be an enemy to your enemies and will oppose those who oppose you. 23 My angel will go ahead of you and bring you into the land of the Amorites, Hittites, Perizzites, Canaanites, Hivites and Jebusites, and I will wipe them out.
We realize that the "angel" in question is not only a divine messanger as opposed to a human prophet à la John-Baptist, but he is just the angel Joshua (!!!) who conquered Canaan going ahead of the Israel's armies.
The rapid inference is that John the Baptist was inserted to eclipse the figure of the previous "angel" who had to come. I can only think that the modification was made because the previous "angel" was too much mythical as figure, even for these writers!