Measuring the Importance of a Mitzvah
The holy sefarim write that one can determine the importance of a mitzvah based on how much opposition he experiences. Because when a mitzvah is very important, the yetzer hara will try harder to prevent him from performing it.
For example, how did Avraham know it was a mitzvah to sacrifice the ram? We do not find that Hashem commanded him to do so. It's because when the ram ran to him, it got caught in the bushes (22:13 בקרניו בסבך נאחז). It was hard for the ram to come to Avraham. That's a sign that sacrificing the ram was a great avodah, and the yetzer hara was trying to prevent it.
Similarly, in Megillas Rus, Naomi tried to convince Rus to return to Moav. But then she changed her mind. This occurred when she saw that the young Rus struggled to keep up with her pace. As it states (Rus 1:18), עליה לדבר ותחדל אתה ללכת היא מתאמצת כי ותרא, "[Naomi] saw that Rus was struggling with all her might to walk beside her, and she stopped speaking with her." The Gra explains that Noami understood from Rus's struggles to walk with her that her resolve was complete. Because when the yetzer hara tries to stop you, that indicates how special the mitzvah is.
The parashah begins with 'ה אליו וירא, And Hashem came to visit Avraham when he was ill (after the milah, see Rashi). The Imrei Emes zt'l explains that Hashem came to see how Avraham was serving Him even while sick. Because some people only serve Hashem when everything goes well for them. Hashem wanted to see Avraham serve Him when he was going through hard times.
