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.

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.

The Bible says this “woman” that sits on the “seven heads” which are “seven mountains” is “that great city, which reigneth over the kings of the earth”:

Revelation 17:18 King James Version (KJV)
18 And the woman which thou sawest is that great city, which reigneth over the kings of the earth.

This “Great City” represented as a woman, is “arrayed in purple and scarlet colour, and decked with gold and precious stones and pearls”:

Revelation 17:4 King James Version (KJV)
4 And the woman was arrayed in purple and scarlet colour, and decked with gold and precious stones and pearls, having a golden cup in her hand full of abominations and filthiness of her fornication:

John is told that there are “seven kings” with five fallen, one currently in power, one “not yet come” but upon arrival will continue a “short space”, and the last “eighth” king who is the same as the current sixth king will rise again and be the last king in this worlds History:

Revelation 17:10-11
10 And there are seven kings: five are fallen, and one is, and the other is not yet come; and when he cometh, he must continue a short space.
11 And the beast that was, and is not, even he is the eighth, and is of the seven, and goeth into perdition.

This very same verse that states “one is” depicts a certain “king” that is currently in power in Johns’ day:

Revelation 17:10
10 And there are seven kings: five are fallen, and one is, and the other is not yet come; and when he cometh, he must continue a short space.

Bible Prophecy uses the words “king” and “kingdom” interchangeably so they have one and the same meaning:

Daniel 2 King James Version (KJV)
44 And in the days of these kings shall the God of heaven set up a kingdom, which shall never be destroyed: and the kingdom shall not be left to other people, but it shall break in pieces and consume all these kingdoms, and it shall stand for ever.

The only “king” or “kingdom” in the time of John was Rome, with the evidence showing the Roman Soldiers mocking, beating, and Crucifying Jesus:

Matthew 27:27-31 King James Version (KJV)
27 Then the soldiers of the governor took Jesus into the common hall, and gathered unto him the whole band of soldiers.
28 And they stripped him, and put on him a scarlet robe.
29 And when they had platted a crown of thorns, they put it upon his head, and a reed in his right hand: and they bowed the knee before him, and mocked him, saying, Hail, King of the Jews!
30 And they spit upon him, and took the reed, and smote him on the head.
31 And after that they had mocked him, they took the robe off from him, and put his own raiment on him, and led him away to crucify him.

Revelation decodes that the current “one is” kingdom is Rome, but there were also 5 previous kingdoms before Rome that had fallen:

Revelation 17:10
10 And there are seven kings: five are fallen, and one is, and the other is not yet come; and when he cometh, he must continue a short space.

The two previous kingdoms before Rome was Greece, and Media Persia:

Daniel 8:15-26 King James Version (KJV)
20 The ram which thou sawest having two horns are the kings of Media and Persia.
21 And the rough goat is the king of Grecia: and the great horn that is between his eyes is the first king.

The third kingdom out of the five kings or kingdoms, is Babylon, and is evident when Daniel tells Nebuchadnezzar that his kingdom is the head of gold from the dream:

Daniel 2:38-39 King James Version (KJV)
38 And wheresoever the children of men dwell, the beasts of the field and the fowls of the heaven hath he given into thine hand, and hath made thee ruler over them all. Thou art this head of gold.
39 And after thee shall arise another kingdom inferior to thee, and another third kingdom of brass, which shall bear rule over all the earth.

The second kingdom known as the previous kingdom before Babylon, was Egypt, as depicted in the Bible when Nebuchadnezzar the King of Babylon annihilated Egypt:

Ezekiel 30:10-11
10 Thus saith the Lord God; I will also make the multitude of Egypt to cease by the hand of Nebuchadrezzar king of Babylon.
11 He and his people with him, the terrible of the nations, shall be brought to destroy the land: and they shall draw their swords against Egypt, and fill the land with the slain.

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.

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