Jude is so named by Luke and Acts. Matthew and Mark call him Thaddeus. He is not mentioned elsewhere in the Gospels, except, of course, where all the apostles are mentioned. Scholars hold that he is not the author of the Letter of Jude. Actually, Jude had the same name as Judas Iscariot. Evidently because of the disgrace of that name, it was shortened to "Jude" in English.
Simon is mentioned on all four lists of the apostles. On two of them he is called "the Zealot." The Zealots were a Jewish sect that represented an extreme of Jewish nationalism. For them, the messianic promise of the Old Testament meant that the Jews were to be a free and independent nation. God alone was their king, and any payment of taxes to the Romans—the very domination of the Romans—was a blasphemy against God. No doubt some of the Zealots were the spiritual heirs of the Maccabees, carrying on their ideals of religion and independence. But many were the counterparts of modern terrorists. They raided and killed, attacking both foreigners and "collaborating" Jews. They were chiefly responsible for the rebellion against Rome which ended in the destruction of Jerusalem in A.D. 70.
As in the case of all the apostles except for Peter, James and John, we are faced with men who are really unknown, and we are struck by the fact that their holiness is simply taken to be a gift of Christ. He chose some unlikely people: a former Zealot, a former (crooked) tax collector, an impetuous fisherman, two "sons of thunder" and a man named Judas Iscariot.
It is a reminder that we cannot receive too often. Holiness does not depend on human merit, culture, personality, effort or achievement. It is entirely God's creation and gift. God needs no Zealots to bring about the kingdom by force. Jude, like all the saints, is the saint of the impossible: only God can create his divine life in human beings. And God wills to do so, for all of us.
"Just as Christ was sent by the Father, so also he sent the apostles, filled with the Holy Spirit. This he did so that, by preaching the gospel to every creature (cf. Mark 16:15), they might proclaim that the Son of God, by his death and resurrection, had freed us from the power of Satan (cf. Acts 26:18) and from death, and brought us into the kingdom of his Father" (Constitution on the Sacred Liturgy).