Soon, the Nation of Israel will be greatly raised up and completely redeemed in the Full Redemption that we eagerly await – and we will all then understand, retroactively, the critical task that Israel played in history during its centuries of Exile.
It will become clear why the Jewish Nation suffered more than the other nations, and why it became a wretched symbol of degradation among the Gentiles. The situation was such that they mocked and ridiculed us by saying: "You claim that you are closer to G-d than anyone else, and yet look at you! The lowest of the low! Doesn't G-d watch over those who are close to Him, and even grant them extra favors? What a disgrace!"
In the future, all such questions will disappear. Everyone will realize that Israel's long-lasting suffering was not because we were lacking, but rather the opposite: Our high stature was the reason for all we went through.
Let us see how the Prophet Isaiah expressed it, beginning with the end of the story: "Behold My servant shall prosper, he shall be exalted and lifted up very high" (52:13). But in the next chapter, the Prophet describes the sharp pain of the Exile: "Despised and rejected, a man of pains and accustomed to illness, as one who hides his face, despised - and we held him of no account. He bore our illnesses, and our pains he carried; we considered him plagued by G-d and oppressed. But he was crushed because of our iniquities; the chastisement of our welfare was upon him, and with his wound we were healed."
That is, the tribulations that the People of Israel suffered were not only because of its own sins. We rather carried the sins of the entire world! For Israel is the heart of the body of nations – and the heart, we know, can either be the most diseased of all the organs, or the very symbol of the body's health. The heart is sensitive and vulnerable; every feeling of fear, worry or sadness, and of love and hate, affects it immediately. Every quick movement, every incident that occurs to one of the other organs – has its influence upon the heart.
And on the other hand, because of its extreme sensitivity, it must itself be perfectly healthy in order to survive. If not, it cannot function. It cannot tolerate any kind of wound, as the other organs can; the other organs are dependent upon it, and it must be constantly sustained at the highest standards.
Rabbi Yehuda HaLevy, in his work Sefer HaKuzari, teaches that just like the heart among the body's organs, so too is Israel among the nations of the world. Israel suffers more than all the others because it bears their sins. Israel is the one who is responsible for all. As G-d tells the Prophet Amos to tell Israel: "Only you have I known among all the families of the earth, therefore I will remember for you and count all your sins."
Yet, still and all, and as a result, G-d will never let the stack of Israel's sins pile up so high that it would cause them to deserve, Heaven forbid, a punishment of total destruction. This is in fact what happened with the Emorites: G-d allowed them to go unpunished for centuries, until they reached the breaking point (see B'reshit 15:16) – and were wiped off the face of the earth. With Israel this will never happen, for G-d punishes them at times, and forgives their sins one at a time, so that they will never pile up.
Thus says the Sage to the King of the Khazars in the above-quoted work: "We suffer when the rest of the world is at peace, but our pains and suffering are meant to strengthen our faith, cleanse the pure ones among us, and remove all dirt and stains from within us – and this brings the world to a state where it can grasp and cleave to the concepts of Divinity." The Kuzari continues that this is the entire idea of the world, to ensure that it cleaves to these concepts – via those who are worthy of such, via the 'heart of the nations,' the Nation of Israel. The reference is specifically to the great men of spirit in Israel, the treasured of the treasure, the prophets and righteous people.
In short, the definition of Israel as the "heart of the nations" indicates, on the one hand, the greatness and singularity of Israel, and on the other, the connection between the Nation of Israel and the other peoples.