It was John the Revelator–the same who authored The Revelation–but he doesn't say it in The Revelation.

"Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time [...] Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son." (1 John 2:18-22)

"And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world..." (1 John 4:3-6)

"For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist." (2 John 1:7)

John very strongly emphasized in his epistles that an antichrist is a deceiver and would lead people away from worshiping Christ.

And I beheld another beast coming up out of the earth [...] And he exerciseth all the power of the first beast before him [...] And he doeth great wonders [...] And deceiveth them that dwell on the earth [...] and cause that as many as would not worship the image of the beast should be killed... (Revelation 11-17)

John made it clear that any who would deceive the people and have them worship anyone but the Saviour was Antichrist. The second beast used great wonders and power to force the people of earth to worship the image of the beast or be killed, and caused all to receive the mark of the beast-a symbol of their worship.

This beast is textbook antichrist. The connection would not have required explanation at the time The Revelation was given to the church.

It was John the Revelator–the same who authored The Revelation–but he doesn't say it in The Revelation.

"Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; [...] Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son." (1 John 2:18-22)

"And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world..." (1 John 4:3-6)

"For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist." (2 John 1:7)

John very strongly emphasized in his epistles that an antichrist is a deceiver and would lead people away from worshiping Christ.

And I beheld another beast coming up out of the earth [...] And he exerciseth all the power of the first beast before him [...] And he doeth great wonders [...] And deceiveth them that dwell on the earth [...] and cause that as many as would not worship the image of the beast should be killed... (Revelation 11-17)

John made it clear that any who would deceive the people and have them worship anyone but the Saviour was Antichrist. The second beast used great wonders and power to force the people of earth to worship the image of the beast or be killed, and caused all to receive the mark of the beast-a symbol of their worship.

This beast is textbook antichrist. The connection would not have required explanation at the time The Revelation was given to the church.