All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
First of all, we praise Allah, Who guided you to Islam, and we ask Him to make us and you firm on it until the day that we meet Him. We also ask Allah to make us among those who invite to His Religion, who act according to His Sharia, and who follow the Sunnah of His Prophet .
As regards your question about what you called obelisks, then it appears to us that by it you refer to the pole or pillar that is erected as a sign on a certain place, such as the one on Mount ‘Arafah.
Your confusion comes from the inaccurate piece of information that you had in mind, that those pillars (obelisks) were symbols for the devil, which is not correct. There is nothing in the Quran or the Sunnah of the Prophet that indicates that these pillars were put as signs for Satan or the devil. The fact that you found this in some books of former scholars or in non-Muslim sciences does not mean that it is the truth. Rather, these are put as marks for determining a specific place and distinguishing it from others, in the same manner that people put marks on their land to shows its boundaries and separate it from the land of the neighbors, for example, or for other purposes.
In the place of throwing the Jamaraat previously, there were pillars, but they were not put as a sign for the devil; rather, they were just marks to specify the place of throwing the Jamaraat, especially for those who are far from it [so that they could see it] due to the large crowd. So the pillar is put to determine the place where the Jamaraat should be thrown.
It is the same thing regarding what you mentioned, that ‘Umar, may Allah be pleased with him, sent four persons from Quraysh to determine the place of the beginning of the Haram and distinguish it from the place that is not included in the Haram and erect pillars there by which people would know that the Haram starts from there. Prior to ‘Umar, may Allah be pleased with him, the Prophet had ordered to renew those marks during the conquest of Makkah.
Ibn Al-Qayyim said in Zaad Al-Ma’aad, “The Prophet ordered Tameem ibn Usayd Al-Khuzaa’i to renew the Ansaab (pillars/signs) of the Haram…”
This is because knowing these boundaries is important as there are many jurisprudence rulings that are related to it.
Imaam An-Nawawi said:
“Knowing the boundaries of the Haram is one of the most important matters that are to be taken into account due to the many (Islamic) rulings that are related to it, and I endeavored to clarify these boundaries and followed the statements of the scholars in being very accurate in determining it, praise be to Allah… And know that there are signs that have been erected in all corners of the Haram. Al-Azraqi and others mentioned with their chains of narrators that Prophet Ibraaheem (Abraham), may Allah exalt his mention, marked the boundaries of the Haram and erected signs around it, and Jibreel (Archangel Gabriel), may Allah exalt his mention, showed him their places, and our Prophet commanded renewing them, then 'Umar, then ‘Uthmaan, then Mu’aawiyah, may Allah be pleased with them, and they are clear until now, praise be to Allah…” [abridged]
With regard to the pillar on the Mount of ‘Arafah, we do not know when it was erected, but it is no more than a mark on the top of the mountain, and there are no Islamic rulings related to it.
Allah knows best.