Praise be to Allah.
It is permissible to speak of Allah, may He be exalted, in terms that do not suggest shortcomings; as for that which suggests shortcomings or may be interpreted in such a manner, it is not permissible to speak of Allah, may He be exalted, in such terms.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
There is a difference between calling upon Him in supplication and speaking of Him. He should not be called upon except by His most beautiful names. As for speaking of Him, that should not be done by using any bad name, but it may be done by using good names or names that are not bad but are also not regarded as sublime, such as the words “Thing”, “Essence” and “Existent”.
End quote from Majmoo‘ al-Fataawa (6/142)
It is good to speak of Allah, may He be exalted, in terms in which it is permissible to speak of Him when there is a need to do so, such as when refuting innovators or when stating something for the sake of argument, or to explain the meaning of the divine attributes, and the like. But one should not expand upon that unnecessarily, because that may lead him to speak of Allah in a manner that is not befitting.
Al-Bukhaari (7418) narrated from ‘Imraan ibn Husayn (may Allah be pleased with him) that some people from Yemen entered upon the Prophet (blessings and peace of Allah be upon him) and said: We have come to you to learn about the religion and to ask you what the beginning of the universe was. He said: “There was Allah and there was nothing before Him, and His Throne was over the water. Then He created the heavens and the earth, and He wrote in the Book all things.”
According to another report: “There was Allah and there was nothing besides Him.” And in yet another report: “There was nothing with Him.”
See: al-Fath (6/289)
If what the one who says that means is to speak of the existence of Allah, may He be exalted, from eternity and that He has always existed and will always exist, and it is impossible for Him to be absent from His sovereignty and power, and it is impossible for there ever to have been a time in past eternity when He did not exist, just as it is impossible that there will ever come a time in future eternity when He will not exist, rather all things will perish save His Countenance as Allah, may He be exalted, says (interpretation of the meaning): “La ilaha illa Huwa (none has the right to be worshipped but He). Everything will perish save His Face. His is the Decision, and to Him you (all) shall be returned” [al-Qasas 28:88] and “Whatsoever is on it (the earth) will perish, And the Countenance of your Lord full of Majesty and Honour will abide forever” [ar-Rahmaan 55:26-27]; and as it is proven that the Prophet (blessings and peace of Allah be upon him) used to say in his du‘aa’ (supplication): “O Allah, to You have I submitted, in You have I believed, in You have I put my trust, to You have I turned in repentance, with Your help have I fought my adversaries. O Allah, I seek refuge in Your glory – there is no god but You – from Your sending me astray, for You are the Ever-Living Who does not die, but jinn and men die.” Narrated by Muslim (2717)…
– If what is meant is in this sense, then there is nothing wrong with speaking of Allah in these terms and describing Him as a Being or Existent or Eternal, may He be glorified and exalted.
Similar things were said by the early generations (the salaf).
It was narrated by Imam Ahmad (10957) and by al-Bayhaqi in al-Asma’ wa’s-Sifaat (1/40) via Ja‘far ibn Burqaan, from Yazeed ibn al-Asamm, that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The people will ask you about everything, until they ask you: ‘Allah created all things, but who created Allah?’” Ja‘far said: Another man told me from Abu Hurayrah (may Allah be pleased with him) – Ja‘far said: He used to attribute it to the Prophet (blessings and peace of Allah be upon him): “If you are asked then say: ‘Allah existed before all things and created all things, and He will exist after all things.’”
It was narrated that Abu Hurayrah said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “Some men will go so far as to say: ‘Allah created the universe, but who created Him?’”
Ma‘mar used to narrate this hadith with a mawsool isnaad and say: “Allah created all things, and He existed before all things, and He will exist after all things.”
Narrated by ‘Abd ar-Razzaaq in al-Musannaf (11/244); he attributed this additional phrase (“He will exist…”) to the Messenger of Allah (blessings and peace of Allah be upon him). Da‘eef
Ibn Abi Haatim narrated in his Tafseer (15664) with a saheeh isnaad from Muhammad ibn Hamzah ibn Yoosuf ibn ‘Abdullah ibn Salaam that when Moosa (peace be upon him) reached the sea, he said: “O You Who existed before all things, and brought all things into being, and will exist after all things, grant us a way out.”
Imam Ahmad narrated in az-Zuhd (345) that Wahb said: Moosa (peace be upon him) said: “O Lord, they are asking me how You began?” Allah said: “Tell them that I existed before all things, brought all things into being, and will exist after all things.”
See also : Madaarij as-Saalikeen (3/183)
But if this wording is applied to Allah, may He be exalted, and the context may suggest some shortcoming or some lack of respect towards Allah, may He be glorified and exalted, that is haraam. It is haraam to say anything that may give the impression of any shortcoming when speaking of Allah, may He be glorified and exalted. But that, as we have said, does not have to do with the original meaning of the word, rather it has to do with what it means in a specific context and what people usually understand from it.
For more information, please see the answer to question no. 48964
And Allah knows best.