what does the Bible mean by those who deny the resurrection of the dead?

There are three New Testament passages that specifically mention denial of a resurrection of the dead:

“There came to him some Sadducees, those who deny that there is a resurrection” (Luke 20:27 ESV)

¹² Now if Christ is proclaimed as raised from the dead, how can some of you say that there is no resurrection of the dead? ¹³ But if there is no resurrection of the dead, then not even Christ has been raised. ¹⁴ And if Christ has not been raised, then our preaching is in vain and your faith is in vain. ¹⁵ We are even found to be misrepresenting God, because we testified about God that he raised Christ, whom he did not raise if it is true that the dead are not raised. ¹⁶ For if the dead are not raised, not even Christ has been raised. ¹⁷ And if Christ has not been raised, your faith is futile and you are still in your sins. ¹⁸ Then those also who have fallen asleep in Christ have perished. ¹⁹ If in Christ we have hope in this life only, we are of all people most to be pitied. …  ²⁹ Otherwise, what do people mean by being baptized on behalf of the dead? If the dead are not raised at all, why are people baptized on their behalf? ³⁰ Why are we in danger every hour? ³¹ I protest, brothers, by my pride in you, which I have in Christ Jesus our Lord, I die every day! ³² What do I gain if, humanly speaking, I fought with beasts at Ephesus? If the dead are not raised, “Let us eat and drink, for tomorrow we die.” ³³ Do not be deceived: “Bad company ruins good morals.”^[d] 34 Wake up from your drunken stupor, as is right, and do not go on sinning. For some have no knowledge of God. I say this to your shame. (1 Corinthians 15:12-19, 29-33)

“But avoid irreverent babble, for it will lead people into more and more ungodliness, 17 and their talk will spread like gangrene. Among them are Hymenaeus and Philetus, 18 who have swerved from the truth, saying that the resurrection has already happened. They are upsetting the faith of some.” (2 Timothy 2:16-18)

Now obviously the Sadducees were not annihilationists, they did not serve God expecting that death is the end of everything, they expected reward as spirits, without the body. The same is true for those that Paul criticises in Corinth “how can some of you say that there is no resurrection of the dead” does not mean they said that there is no hope, no reward, no life after death, but they were holding to the teaching less common among Jews but almost universal among Greeks of an immortal soul, or spirit world after death. The same is true of Hymenaeus and Philetus who would be teaching something similar to the Sadducees and Greeks. Not that they Hymenaeus and Philetus were teaching no hope, no spirit life, but no resurrection. 

To deny the resurrection = heaven going

It’s clear from early Christian comments, most strikingly the following, that those who came to deny mortalism and the resurrection of the dead in the early church did not teach annihilation, but immortalism, souls going to heaven.

“who say there is no resurrection of the dead, and that their souls, when they die, are taken to heaven; do not imagine that they are Christians,….” (Justin Martyr c130 AD)

Despite men like Justin strongly teaching mortalism and the death and resurrection of man this became the norm in the church in the time leading up to the establishment of the Trinity at Nicea in 325AD. Clear teaching on mortalism did not resurface in the Western Church until the reformation with the radical reformers. Leading Jean Calvin to write his first work ‘Night Vigil of the Soul’ or in Latin title Psychopannychia, 1534, which opposes the mortalism, characterised by Calvinists as “soul sleep”, as taught by Anabaptists and other radical Protestants.