Is belief in qadar (destiny) one of the essentials of being a Muslim?

It is one of the signs of possessing faith (iman). Umar ibn al-Khattab narrated: "While we were sitting with the Messenger of Allah, may Allah bless with him and grant him peace, one day a man came up to us whose clothes were extremely white, whose hair was extremely black, upon whom traces of traveling could not be seen, and whom none of us knew, who sat down knee-to-knee with the Prophet (saw). [The man] said, 'Muhammad, tell me about Islam.' The Prophet (saw) said, 'Islam is that you witness that there is no god but Allah and that Muhammad is the Messenger of Allah, and you establish the prayer, and you give the Zakat, and you fast Ramadan, and you perform the hajj to the House if you are able.' He said, 'You have told the truth,' and we were amazed at him asking him and [then] telling him that he told the truth. He said, 'Tell me about Iman.' He said, 'That you affirm Allah, His angels, His books, His messengers, and the Last Day, and that you affirm the Decree [predestination], the good of it and the bad of it.' He said, 'You have told the truth.' He said, 'Tell me about Ihsan.' He said, 'That you worship Allah as if you see Him, for if you don't see Him then truly He sees you.' He said, 'Tell me about the Hour.' He said, 'The one asked about it knows no more than the one asking.' He said, 'Then tell me about its signs.' He said, 'That the female slave should give birth to her mistress, and you see poor, naked, barefoot shepherds of sheep and goats competing in making tall buildings.' He went away, and I remained some time. Then he asked, 'Umar, do you know who the questioner was?' I said, 'Allah and His Messenger know best.' He said, 'He was Jibril who came to you to teach you your deen'." (Sahih al-Muslim)