Genèse 28:18

And Jakob arose in the morning, and took the stone which he had placed for his pillow, and set it standing, and poured oil on the top of it.

AND HE SET IT UP FOR A PILLAR. Our Rabbis have explained the difference between a pillar and an altar by saying that a pillar consists of one stone while an altar is composed of many stones. It further appears that a pillar is made for pouring libations of wine upon it and for the pouring of oil upon it, but not for sacrifices and not for offerings, whereas an altar is for bringing Burnt-offerings and Peace-offerings thereon. When Israel entered the Land, the pillar was prohibited to them because the Canaanites had established it as an ordinance of an idolatrous character to a greater extent than the altars. Even though it is written concerning the altars, Ye shall break down their altars, [since the altars were not as prevalent as pillars among the Canaanites, He did not prohibit the Israelites from making their own altars]. It may be that He did not want to prohibit all sacrifices, and so He retained the altar as fit for libations and sacrifices.

ויצק שמו על ראשה, he anointed it to sanctify it and to make it ready to offer sacrifices on it when he would return to that site. We have such formulations in connection with the sanctification of the various materials donated for the Tabernacle (Numbers 7,1) as well as in Genesis 31,13 and again in verse 22 in our chapter here.

וישם אותה מצבה, he sanctified it as an altar which he would erect on his return from Charan. The Torah testifies in Genesis 35,14 that this is indeed what Yaakov did on his way home to the land of Canaan. ויצב יעקב מצבה במקום אשר דבר “Yaakov established an altar on the site where he had said.”

וישכם יעקב....וישם מצבה, he erected the stone to its full height.

ויצק שמן על ראשה, so that he would recognise this stone on his return from Charan. Oil stains on a stone remain for many years and do not disappear. He planned to erect an altar at this site when he would return from Charan. It is also possible that the pouring of oil on that stone was equivalent to pouring a libation of wine on the altar. In fact, when he did return to this site he is reported as doing exactly this (Genesis 35,14) All of this was to be a visible reminder that the Jewish people would take possession, inherit the land promised to them by G’d. Just as Avraham and Yitzchok during their time had treated the land as if it already belonged to them, Yaakov now did the same, albeit it in a miniscule, symbolical way. Once Yaakov had made a monument out of this stone no one had the right to use this stone for his private needs. The fact that some 34 years later he built an altar there using this stone as its foundation, shows that no one in the interval had disputed what this stone had represented. The Torah would not bother to mention that Yaakov had called this site Bet El unless the local people had been informed of this and had not objected to the name change. Clearly, just as the local population had to go along with the wells dug by Avraham and Yitzchok and the names given to these wells because G’d made them respect these patriarchs, so now G’d inspired the local residents to go along with what Yaakov had done there. The attitude G’d inspired in the local population is best described by David in Psalms 105 15 אל תגעו במשיחי ולנביאי אל תרעו, “do not dare touch My anointed, nor do harm to My prophets.”