In Parshas Lech Lecha, G-d appears to Avraham after the battle with the kings, and says to him, "Anochi Magen Lach", "I am your shield."
We incorporate this three times into our daily prayers (four times on Shabbat) in Shmone Esre, in the very first of the eighteen brachot, remembering our three forefathers (avot) Avraham, Yitzchak and Yaakov. But the concluding blessing only uses one of them, Magen Avraham, Shield of Avraham. Why only Magen Avraham?
This particular blessing is so important that failing to concentrate on it requires repeating the Shmone Esre. What of the other Avot? What is so special about Avraham and about G-d's role as Magen Avraham, Shield of Avraham?
Let us look at the three descriptions leading up to it of G-d as, Melech Ozer, Moshia UMagen, the King who Aids, Saves and Shields. Each of these praises rises in stature, as is appropriate when praising G-d, by going from the lesser to the greater.
G-d as the one who aids us is the least of these. What does that mean? If we're in trouble, we call out for help, and G-d helps us get through the difficult situation. For example we're driving a car that skids out of control on a wet road, and with G-d's help, the accident isn't too bad, and after a few weeks of physical therapy, things are back to normal. We praise G-d for aiding us, because if not for Him, we would be dead or crippled.
Then there's G-d as the one who saves. So when we find ourselves in trouble, we call out to Him, and he saves us from our troubles without a scratch. So when that same car begins to slip, it pulls to a stop, with no harm done.
Then finally there's the G-d who shields. And when He shields us, we never even experience the crisis. We just drive on, never even experiencing any portent of danger. Because G-d is shielding us all the way. Now obviously this is the ideal experience. And that is why the G-d who shields is the highest level of praise.
But Shield of Avraham also requires the highest level of faith. When we experience G-d as aiding us through a difficult situation or saving us when we're in danger, it's easy to see His handiwork and know that He's protecting us. But when He's shielding us, it takes a lot of faith to remember that when nothing goes wrong, it's because of G-d acting as the Shield of Avraham.
Now let's look at each of the three Avot and how they match up to these three praises. Avraham was shielded by G-d. As a result he was never faced with any real physical threat. When he fights the kings, G-d appears afterward to tell him that he is being shielded. There is no mention of any actual danger.
On the other hand, Yitzchak does experience imminent physical danger at the Akeidah, but he's saved from it at the last moment. He experiences Hashem as Moshia. Yaakov not only experiences physical danger, he actually experiences physical harm. He's crippled by the angel, his children are abducted twice, but Hashem as the Ozer, helps him get through these troubles.
Yet if we look at the reward that G-d promises Avraham, when He tells him that "Anochi Magen Lach, Sharcha Harbeh Meod", "I am your shield, your reward will be very great", it is a long way off. Because the reward is Yitzchak, and Avraham has to wait for most of his life for that. Yitzchak has to wait for children, but less time than Avraham did. Yaakov has to wait the least amount of time (depending on which dating system you use) for children.
That is because Emunah Schar Emunim, Faith is the Reward of the Faithful. Because Avraham merited to experience G-d as a shield through his high level of faith, he was also given the privilege of living by that faith and waiting to see G-d's promise fulfilled. As it says in Habbakuk, VeTzaddik BeEmunotoh Yihyeh, The Righteous Shall Live by his Faith. Each of the forefathers embodied that in a different way.
When we recite Magen Avraham in Shmone Esre, we are reminded that we should be thankful not only for the times that G-d visibly saves or aids us, but for His shield over us, as it says later in Magen Avraham, Al Nisecha SheBekol Yom Imanu, For Your miracles that are with us daily. And so we aspire to the faith of Avraham, while praising G-d as the Shield of Avraham, who protects us from harm and peril even when we do not realize it. And if we forget, we are obligated to go back and recite the verse again, to be reminded of something that is very easy to forget, and yet so very important to remember.