Is the Roman Catholic Church the 1st Beast of Revelation?

Yes.

Revelation tells us that John is in vision, that he is on the sand of the sea and sees a Beast rising up out of the sea:

Revelation 13:1 King James Version (KJV)
1 And I stood upon the sand of the sea, and saw a beast rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads the name of blasphemy.

Throughout the Bible, the sand of the sea represents innumerable multitudes of people:

Genesis 32:12 King James Version (KJV)
12 And thou saidst, I will surely do thee good, and make thy seed as the sand of the sea, which cannot be numbered for multitude.

Revelation plainly tells us that that these Waters from which the Beast arose from are many people, multitudes, nations, and languages:

Revelation 17:15 King James Version (KJV)
15 And he saith unto me, The waters which thou sawest, where the whore sitteth, are peoples, and multitudes, and nations, and tongues.

This Beast has Seven Heads and Ten Horns:

Revelation 13:1 King James Version (KJV)
1 And I stood upon the sand of the sea, and saw a beast rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads the name of blasphemy.

Revelation 17 writes that the Seven Heads represent Seven Mountains:

Revelation 17:9 King James Version (KJV)
9 And here is the mind which hath wisdom. The seven heads are seven mountains, on which the woman sitteth.

Revelation 17 reveals that the Ten Horns are Ten Kings:

Revelation 17:12 King James Version (KJV)
12 And the ten horns which thou sawest are ten kings, which have received no kingdom as yet; but receive power as kings one hour with the beast.

Daniel is shown that another King shall rise after the Ten Kings, and be different to all of them, and will subdue Three Kings:

Daniel 7:24 King James Version (KJV)
24 And the ten horns out of this kingdom are ten kings that shall arise: and another shall rise after them; and he shall be diverse from the first, and he shall subdue three kings.

The Ten Kings were the Ostrogoths, Visigoths, Franks, Vandal, Suevi, Alamanni, Anglo Saxons, Heruli, Lombards, Burgundians.