On the words משמרתי וישמר, Ramban writes, "This means Avraham guarded the emunah in his heart, he fought with idol worshippers, and he called out Hashem's name to draw many people to His service."
The Siforno (26:5) says that Avraham's deeds of chessed are alluded to from the words משמרתי וישמר, "[Avraham] performed My mission." The Siforno explains that it is Hashem's mission and will to do kindness to people. Avraham was occupied in chessed, and it was like he was performing the chessed that Hashem would have done. The Siforno writes, המיוחדת המשמרת תמיד עשה חסד לגמול שהיא לי, "Avraham always performed the service that is Mine, and that is to do kindness..."
The Siforno proves that it is Hashem's way to do chessed from the pasuk (Tehillim 25:10) ואמת חסד 'ה אורחות כל, "All the ways of Hashem are kindness and truth..."
The greatest kindness you can do for a person is to teach him emunah. If you help someone attain emunah, you did him a great favor in the next world because he will earn Olam HaBa. But you also gave him goodness in this world because no one is happier in this world than someone who believes in Hashem. The believer knows that everything is for the good; he can speak with Hashem whenever he has a need, and he can become close to Hashem by performing the mitzvos.
The Rambam (Avodah Zarah 1:3) writes, "As soon as this strong one (Avraham) was weaned, his mind began to search. He was young, and he was thinking day and night. He wondered, 'How can it be that the sun is always moving, and no one is turning it...? He didn’t have anyone to teach him. He lived in Uhr Kasdim among stupid idol worshipers (הטפשים כוכבים עובדי בין). His father, his mother, and the entire nation worshiped the stars, and he worshiped the stars together with them. But his heart was searching... until he perceived the true path... He knew that there is one G-d; He directs the sun; He created everything; and there is no G-d other than Him. He knew that everyone in the world was mistaken. He was forty years old when he recognized his Creator. Avraham began...debating with the people of Ur Kasdim. He told them, 'You aren't on the right path.' He broke their idols and told them that one should serve solely the Creator of the world... Avraham brought many proofs, and the king wanted to kill Avraham. A miracle happened [and his life was spared]... Avraham announced to everyone... that there is only one G-d, and we should serve Him alone... Tens of thousands followed Avraham, and he planted in their hearts this great truth. He wrote sefarim and taught it to his son, Yitzchak."
Avraham taught people about Hashem, and that is the greatest chessed. Avraham's calling out in Hashem's name is written at the beginning of Lech Lecha (Bereishis 12:8), 'ה בשם ויקרא 'לה מזבח שם ויבן, "He built there a mizbeiach to Hashem and called out in Hashem's name."
The Ramban on that pasuk writes, "Unkelos explains that 'ה בשם ויקרא means that Avraham davened there... But the correct translation is that Avraham called out Hashem's name in a loud voice before the mizbeiach, and he taught people about Hashem. He would do so [when he lived] in Ur Kasdim as well, but then people didn't want to listen to him. But now that he came to this land that he was promised ואברכה מברכיך, that he would be blessed, he was now able to teach and to reveal Hashem to people."
The Ramban continues, "When Yitzchak went to Gror...it states 'ה בשם ויקרא, that Yitzchak called out and taught people about Hashem. This is because he came to a new place, and they didn't hear about Hashem... Yitzchak revealed Hashem's honor to the nations. Regarding Yaakov, it doesn't say he called out in Hashem's name. This is because Yaakov had many children, all of them were Hashem's servants...and the emunah in Hashem was revealed through them... Furthermore, everyone in Eretz Canaan already knew Hashem from the work of his father and grandfather. As it states in the Midrash (Bereishis Rabba 39:16), 'Avraham had every creation call out in Hakadosh Baruch Hu's name, as it states (Bereishis 121:8) 'ה בשם ויקרא."
The Siforno writes that when Yitzchak began calling out in Hashem's name and teaching people emunah, he no longer had any troubles in his life. Previously, he suffered from people's jealousy and theft, but when he began teaching people about Hashem, his life became better. "After he called out Hashem's name, Avimelech said to him (26:28-29) עתה אתה ...עמך 'ה היה כי ראינו ראה 'ה ברוך, 'We have seen that Hashem is with you... you are now the blessed of Hashem.'"
