In response to the question you raised about whether Saudi Arabia is an Islamic State (Dar al Islam) or not, we need to first define the meaning of Dar al-Islam and Dar al-Kufr (Land of Kufr) and then look to whether Saudi Arabia or any Muslim country today matches this definition.
The following article explains this subject in detail together with the evidences, please check it: Clarfiying the meaning of Dar al-Islam and Dar al-Kufr
1) According to Shariah terminology, Dar al-Islam is defined as the land which is governed by the laws of Islam and whose security (Aman) is maintained by the security of Islam, i.e. by the authority and protection of Muslims inside and outside the land, even if the majority of its inhabitants are non-Muslims.
Dar al-Kufr is the land which is governed by the laws of Kufr, and whose security is not maintained by the security (Aman) of Islam, i.e. by other than the authority and security of Muslims, even if the majority of its inhabitants are Muslims.
So what matters in determining whether the land is Dar al-Islam or Dar al-Kufr is neither the land itself nor its inhabitants, rather it is the laws and the security. So if its laws are Islamic and its security is maintained by Muslims then it is Dar al-Islam. When its laws are the laws of Kufr (disbelief) and its security is not maintained by Muslims then it is Dar al-Kufr. These definitions have been derived from the Islamic evidences and discussed by the Ulema (scholars) in history.
2) When we look to Saudi Arabia it fundamentally contradicts Islam in a Qati (definitive) manner in many areas such as:
- Being part of the United Nations - every member of which has to agree with international law set by the UN. This puts the law of man above the law of Allah.
- Allying with the Kuffar against the Muslims. Saudi Arabia allowed America to use it to attack the Muslims of Iraq in the Gulf wars. America also has a military base in Saudi which is well known.
- Accepting the legitimacy of the different countries in the Muslim world this is clear by the fact they are part of the OIC and Muslim League and have ambassadors in these different countries. This is definitively prohibited from different ahadith and the Ijma of the Sahaba:
The Prophet (saw) said: "When the oath of allegiance has been taken for two Khalifs, kill the latter of them". (Narrated in Sahih Muslim by Sa'id al-Khudri)
The Prophet (saw) also said: "Whoso comes to you while your affairs has been united under one man, intending to break your strength or dissolve your unity, kill him." (Narrated in Sahih Muslim by 'Arfajah)
3) Just because Saudi Arabia may implement part of Islamic laws such as in the judiciary, it doesn’t mean that it is an Islamic state or Khilafah
"So rule between them by that which Allah has revealed, and follow not their desires, but beware of them in case they seduce you from just some part of that which Allah has revealed to you" (TMQ 4:49).
"Then it is only a part of the Book that you believe in and do you reject the rest? But what is the reward for those among you who believe like this but disgrace in this life and on the day of judgement they shall be consigned to the most grievest chastisement." [Al-Baqarah: 85]
4) We cannot compare Saudi Arabia and say it is similar to the Khilafah after the Khulafah ar-Rashideen. As in those periods it is unanimously accepted by the Sunni Ulema in history that they were Khilafah and Dar al Islam, some of them misapplied some of the Islamic rules but they never implemented open kufr. There is a difference between the mis-implementation of some of the Shariah rules and the non-implementation as we see in Saudi Arabia.
Some of the Khulafah were better than others and some of them were even oppressive, nevertheless the system of Islam remained implemented. Even though some of the rulers misapplied some rules and were oppressive, the Prophet (saw) foretold of this and instructed the Muslims to accept them as Khulafah and obey them as long as they did not implement open Kufr (Kufr bu’ah). Some of these ahadith are:
Muslim reported that Salama b. Yazid Al-Ja‘afi asked the Messenger of Allah (saw): "O Prophet of Allah, if we were to be ruled by Ameers who ask us for their dues and deny us our dues, what do you order us to do then?" The Messenger of Allah(saw) turned his face away; he asked him again and Allah’s Messenger (saw) avoided him; then he asked for the second or the third time and he (saw) was pulled by Al-Ash‘aath b. Qays, so the Messenger of Allah (saw) said: "Hear and obey, for they shall be accountable for their actions and you shall be accountable for yours."
Muslim reported from ‘Auf b. Malik who reported: I heard the Messenger of Allah (saw) say:"The best of your Imams are those whom you love and they love you and you pray for them and they pray for you; and the worst of your Imams are those whom you hate and they hate you and you curse them and they curse you." We asked: "O Messenger of Allah, shall we not then declare war on them?" He (saw) said: "No! As long as they establish prayer among you. Behold if anyone was ruled by a Wali and saw him committing a sin, let him hate the sin committed against Allah, but let him not withdraw his hand from obedience."
Muslim narrated from Huzayfah b. al-Yamaan that the Messenger of Allah (saw) said: "There will be Imams after me who will not be guided by my guidance, nor will they act according to my Sunnah; some men will rise amongst you with satans’ hearts in human bodies." Huzayfah asked: "What shall I do, if I were to reach that time?" He (saw) said: "You should hear and obey the Ameer even if he whipped your back and took your money; do hear and obey."
Ahmad and Abu Dawood reported that the Messenger of Allah (saw) said: "O Abu Dharr, what would you do if some Walis possessed the booty and deprived you of it?" He said: "By He Who sent you with the Truth, I would raise my sword and fight until I join you." Upon this he (saw) said: "Let me tell you something that would be better for you than that. Remain patient and bear it until you join me."
Al-Bukhari narrated on the authority of Junada b. abi Umayyah who said: We went to ‘Ubadah b. as-Samit when he was sick and we said: May Allah (swt) guide you. Inform us of a Hadith from the Messenger of Allah (saw) so Allah may benefit you from it. He said, the Messenger of Allah (saw) called upon us and we gave him the Bai’ah, and he said, of that which he had taken from us, that we should give him the pledge to listen and obey, in what we like and dislike, in our hardship and ease, and that we should not dispute the authority of its people unless we saw open Kufr upon which we had a proof from Allah. And it was also narrated by Tabarani. He said: “Unless you see open Kufr.”
The Khulafah ruled by Islam and did not implement Kufr Bu’ah (open Kufr) that is why we find the great scholars of Islam such as Imam Abu Hanifah, Imam Shafi, Imam Malik, Ahmad ibn Hanbal, Jafar as-Sadiq and the like accepted that they were Khalif’s but accounted them. In fact the two most famous students of Abu Hanifah who recorded his Fiqh, most of us in India follow the Fiqh they recorded - Qadi Abu Yusuf and Muhammad As-Shaybani were appointed as Qadi al Quda (the chief judge) in state in the time of the Khalifah Haroon ar-Rasheed.
Some of the great scholars wrote books about the history of the Khulafah such as Imam Suyuti known as al-Jalalayn, who died in 1505 CE – he wrote the book Tarikh al Khulafah, ‘History of the Khulafah’. Imam Mawardi who died in 1058 CE who was appointed as a Qadi and later an ambassador by the Abbasid Khalid Al-Qaim bi Amr Allah wrote the famous book, ‘Al Ahkam As-Sultaniya’, about the ruling system in Islam.
Whereas today all the countries in the Muslim world including Saudi Arabia implement Kufr Bu'ah in many areas such as those mentioned earlier.