Will believers who populate the New Earth be able to sin? I know the 2nd generation will (they’re not technically believers until they accept Salvation, just like the rest of us), but how about the 1st generation? The ones who were left on Earth at the start of the Millennium?
Believers who survive the Great Tribulation will go into the Millennial Kingdom in their natural state. That means they’ll have a sin nature and will be unable to keep themselves totally free from sinning. That’s why in Israel daily sacrifices for sin will begin as soon as the Temple is ready for use (Ezekiel 46:13)
However, given their experiences I would imagine the frequency of their sins would be much lower than we see around us now. As successive generations are born I expect the frequency will steadily increase, as more and more of them reject the Lord’s offer of pardon. Otherwise where would Satan get the massive army he’ll recruit for one last attempt to overthrow the Lord (Rev. 20:7-8)?