The belief that the existence of a caliph or any other ruler is not necessary in religion - if members of society are able to maintain relations based on religious principles, then they may not need a ruler.
Allowing the concealment of their true beliefs, if they were in territories where the Sunnisdominated.
Sinning Muslims were not excommunicated as unbelievers. The Najdat believed that they could be forgiven by Allah - only he who persisted in his sin and repeatedly committed it, could be accused of unbelief.
History
Najda revolted in Yamamah, following the death of Husayn ibn Ali in 680. In 683 he allied his cause with the anti-caliph Abd Allah ibn al-Zubayr, helping to lift the Umayyadsiege ofMecca. In 684 Najda went to Basra and, led by Nafi ibn al-Azrakh, participated in the Kharijite rebellion against the governor Muslim ibn Uways, who was about to attack the city. The Azariqa withdrew to Ahwaz, but Najda separated and returned to Yamama. In 685, after some particular successes, the Kharijites recognized Najda as their leader. In 686 he went to Bahrayn and settled in Qatīf, which he made the Najdat capital. After a failed attempt to capture Basra, Najda conquered Oman but was driven out by a local revolt, passing Oman to the two sons of Abbad ibn Abd-Allah. In 687 a group led by Atiyya ibn al-Aswad split up and headed to Sistan where they founded their own Kharijite branch. In the same year, Najda invaded Yemen and seized it, including Sanaa, and his lieutenant Abu-Fudayk conquered the Hadramaut. The Najdat now controlled the most peripheral areas of the caliphate and therefore most adequate to avoid the oppression of caliphate forces. The Najdat then began their conquest of Hejaz where, after the defeat of Abd Allah ibn al-Zubayr, they suffered an attack from Muhammad's cousin Abd Allah bin Abbas. Here they blocked supplies to Mecca and Medina and isolated Taizz, but did not want to attack the holy villages. At this point the Najdat dominated almost all of Arabia. However, an ideological split severed their ranks, between those who favored the continuation of the fight against the Umayyad "usurpers" and those who were in favor of a treaty with Damascus. Subsequently, some of Najda's supporters began to object to certain beliefs of his and rebelled against him. The intransigents, led by 'Atiya al-it Hanafī, took refuge in the Iranian region of Helmand, assuming the title of Atawiyya, while some more radical Najdat, led by Abu Fudayk, murdered Najda himself in 691 and took his place. They later tried to fight against the Umayyad caliphAbd al-Malik ibn Marwan. In 692 Fudayk repelled a caliphate attack from Basra, but they were eventually defeated by the caliphate in 693, at the battle of Mushahhar. The free territory of the Najdites collapsed, although some of the supporters of Najda still remained faithful to his teachings. For some time, Najdism continued to exist, remaining in practice until the 11th century.