ani HASHEM Elokei Avraham avicha ve'Elokei Yitzchak - I am HASHEM, G-d of Avraham your father, and G-d of Yitzchak
G-d here assures Yaakov that He will give the land to Yaakov and his descendant, that he will be with him and will protect him in all his undertakings, similar to Tehillim 91:11 "for He will command His angels to protect you on all your paths." The reason that the pronoun ani is used is to show that the angel who appeared to Yaakov wanted him to understand that he was acting as G-d's messenger, i.e. was part of the Divine establishment. It is as if G-d had said, "I am your share and your heritage" (BaMidbar 18:20) The Kohanim were informed that they are an integral part of the Divine. Similarly, we have a statement (Devarim 32:9) describing the whole Jewish nation in similar terms, i.e. "For His people are part of G-d, Yaakov is a portion of His heritage," and not merely a portion of the angels which had been mentioned.
This paragraph contains both an assurance to Yaakov personally at this time that G-d will be with him on the journey he had undertaken and that He would bring him safely back to his father's house, but G-d also gave him an assurance regarding his offspring. G-d would be with them also wherever they would be even if at times they would be in exile just as Yaakov at this time. Our Sages in Megillah 29 expressed it thus: Wherever the Jewish people will be exiled the Shechinah will be exiled with them. The Shechinah would look after the Jewish people and protect them from many troubles they would otherwise experience while in captivity.
When G-d said "I am Hashem, G-d of Avraham your father and G-d of Yitzchak" He mentioned two attributes. The attribute gadolah and the attribute of gevurah. Remember that the site upon which Yaakov was dreaming was Beit-El, a site of which Yaakov was to say when he woke up, "this must be a House of G-d, and hence the gateway to heaven." The place was also known as Luz. It is not the same Beit-El as the one which is located near the town of Ai which Yehoshua had trouble capturing and near which Yervo'am placed one of his golden calves to prevent his subjects from making the pilgrimage to the Temple in Yerushalayim (1Melachim 12:29)
Yaakov named the site Beit-El (v19) because this is where he had a vision of G-d. This verse does not mean to conceal something but to explain something. It explains such statements as elokim kedshim (Yehoshua 24:19), and elokim shoftim (Tehillim 58:12), where it appears at first glance as if the plural ending applied to G-d as "Holy" or "Judge," indicates that there is more than one G-d. Concerning this revelation to Yaakov that G-d appeared to him as two attributes Yeshayahu said, "serafim are standing above him" (Yeshayahu 6:2) It was a reference to two attributes of G-d. These two attributes of G-d are particularly manifest at Beit-El. This was also the reason that the hertic King Yervo'am was careful to erect not one golden calf but two golden calves. The Jewish people in the desert had made one golden calf for themselves when Moshe did not return when they expected him. They had thought that he was dead and they felt they needed a new symbol to replace Moshe and to serve as their visible inspiration. They were fully content with a single leader.
Yervo'am had a different motivation. He wanted to counteract the Divine attribute of gedolah and hoped that the presence o the golden calf, something G-d hated, would drag away spiritually positive influences and forces from Yerushalayim, i.e. from the Temple. The second golden calf which Yervo'am made was intended to counteract the Divine attribute of gevurah, as he wanted to be the stronger of the two kings ruling over Yisrael. This attribute is the specific attribute with which a king ought to be endowed. He hoped that by weakening the presence of this attribute in Yerushalayim he would thereby strengthen his own hold on that attribute. The reason that he placed both golden calves outside of Yerushalayim was that he wanted to undermine Yerushalayim as "the city of David." Yerushalayim was also known as "the city of righteousness" (Yeshayahu 1:26) The power of the Kingdom of David was concentrated in that city as the attribute of righteousness is manifest in that city. Tzedek, righteousness was the outstanding attribute, the virtue of David. We know this from Yeshayahu 32:1 "A king shall reign in righteousness" (the king being the Mashiach, a descendant of King David). Yervo'am was intent on nullifying this monarchy. It had been made even stronger both symbolically and spiritually, seeing that the Temple in Yerushalayim was "opposite" the celestial Temple. All these considerations prompted Yervo'am to distance himself from the site of the Holy Temple and the holy city Yerushalayim.
When Sefer Melachim proceeds to describe where these two golden calves were placed, i.e. one at Beit-El and one in Dan, he had clearly violated the mitzvah "do not make beside Me silver or golden deities" (Shemot 20:23). However, the worst part of his sin was that he portrayed what he had done as a shortcut to heaven, wanting to make his people believe that now that they had symbols of higher emanations than were to be found in Yerushalayim there was no longer any point in making the trip to the Temple in Yerushalayim. Sefer Melachim testified to Yervo'am's real concern namely that when the people would make the pilgrimage to Yerushalayim it would not take them long to find out that they had been fooled by their king, and they would then kill him.