Mirza Abu'l Fadl about Shi'i Imams

You know that after the ascent of Jesus into the heavens of God's presence (or, to put it more clearly, from the time these revealed prophecies descended) until the dawn of the morn of the Day of God, no one arose with prophethood or guardianship to bear witness to the unity of God and to the truth of His prophets and messengers, from Adam to Christ, save our Lord Muhammad. And the gate to the city of his knowledge was `Ali. These two resplendent luminaries, these blessed olive trees, arose to proclaim the unity of God and the truth of His Messiah. Through their witness, great nations became believers, though they had not formerly recognized the oneness of God or the truth of Christ Jesus, the Spirit of God, but rather worshiped fire and idols. They came to believe and obey; they assented and attained certitude.

In this way the divine revelation was fulfilled, and the vision of John the Divine became a reality. After they completed their witness, their light spread and their word was established. Then appeared the frightful beast, that is, the vicious monarchy, the despotic caliphate of the Umayyads. The leadership of this Qurayshi clan, the kings of the Umayyad caliphate, fought against them. Abu Sufyan made war on the Prophet; Mu'awiyah contended with `Ali and poisoned Hasan; Yazid waged war with Husayn; Hisham ibn `Abdu;l-`Aziz struck at Zaynu'l-`Abidin, the son of Husayn. Thus, every single member of this pure, prophetic clan and this blessed, `Alid family was poisoned or killed by the order of one of these tyrannical Umayyad caliphs.

When Yazid died, `Ali Zaynu'l-`Abidin, the son of Husayn, returned to Medina. His presence restored the imamate, and the doors of guidance were flung open a second time upon his companions. The Umayyad and `Abbasid caliphs always feared the Prophet's family, and they watched and spied on them. They harassed them one by one until 260 A.H. [874 A.D.], when the imamate ended.

http://bahai-library.com/gulpaygani_miracles_metaphors&chapter=all