Who are the Nicholaitans mentioned in the Book of Revelation, and what are the works Jesus and the believers in Ephesus hate?
Nicolas was a proselyte of Antioch. Irenaeus [130-202 A.D.] was of the opinion that he was the founder of the Nicolaitans.
Acts_6:5 And the saying pleased the whole multitude: and they chose Stephen, a man full of faith and of the Holy Ghost, and Philip, and Prochorus, and Nicanor, and Timon, and Parmenas, and Nicolas a proselyte of Antioch:
The following is extracted from Wikipedia.
Several of the early church fathers mentioned this group, including Irenaeus, Hippolytus, Epiphanius, and Theodoret, stating that deacon Nicolas was the author of the heresy and sect. Irenaeus in Adversus Haereses III. xi. 1; I. xxvi. 3 holds that the Gospel of John was written to counter the teachings of Cerinthus, which he holds was spread by the Nicolaitans. But when Irenaeus focuses on them later, he only presents them as the Book of Revelation did, with no explanation how they can be held to have the doctrines of Cerinthius. "The Nicolaitanes are the followers of that Nicolas who was one of the seven first ordained to the diaconate by the apostles. They lead lives of unrestrained indulgence. The character of these men is very plainly pointed out in the Apocalypse of John, [when they are represented] as teaching that it is a matter of indifference to practice adultery and to eat things sacrificed to idols."— Irenaeus, Adversus haereses, i. 26, §3
Hippolytus of Rome shared the opinion that Nicolas became a heresiarch (in Refutation of All Heresies vii. 24). In other writings of the early Church, this connection is disputed and the Nicolaitans are said to be "falsely so called" (ψευδώνυμοι). Clement of Alexandria put forward a defense of Nicolas (in Stromata ii. 20, iii. 4) which Eusebius accepts and repeats (in Historia Ecclesiastica iii. 29).