Iron, with its superior strength, represents a powerful and stubborn Kelipah which could not be refined in earlier generations. And so it could not be incorporated into the Beis Hamikdosh.
In a vision, Daniel saw a statue which represented the exiles which the Jewish people would suffer. Edom (Rome), the force which destroyed the Beis Hamikdosh and began this long bitter exile, was depicted as legs of iron.
Iron can only be defeated with iron.
By accessing the Barzel of Kedusha we can refine and nullify the strong Kelipah of Barzel, the iron of Edom
The iron of Kedusha lies in the essence of the Neshama. The Jewish people are called a stiff-necked people. In a positive sense, this refers to our ability to be stubborn in standing unbending and uncompromising in our service Hashem, even when facing the stubborn obstructions from within and without.
In the final generation of Golus, we can access the iron of the Etzem Haneshama. Using this powerful force of Kedusha, we will overcome the iron of Edom. The iron legs on which Golus stands will crumble.
And when it does, the eternal Beis Hamikdosh Hashelishi will be built using iron – the Barzel of Kedusha.