This statement could be misleading. The Revelation seems to have been considered as Scripture by some in the 2nd Cent. The canon wasn't set by a single vote. There was a long process of discussion in the Church. Revelation was one of the books about which there was the most controversy. But by the 4th Cent it was generally accepted, perhaps later in the East. I'm not convinced that the Council of Carthage had any particular authority with respect to the canon.
The Revelation has continued to be a bit marginal. According to the Wikipedia discussion on the canon, it has never been in the Greek lectionary. It's one of the few books that Calvin didn't do a commentary on and didn't refer to much if at all. Luther considered it (together with a few other books) as dubious.
Revelation was written in the late 1st Century AD and admitted into the canon in the late 4th Century AD.
It has been speculated and debated upon ever since.