During the 1st century Judah was a common name among Jews, and when Jesus called 12 men to become his apostles, he chose two with the name Judah. Based on the original Greek text this was rendered in Latin as Judas and for many centuries there wasn’t a linguistic separator between Judas Iscariot and Judas Thaddeus.
As a result, early medieval Christians normally did not pray to the apostle Judas Thaddeus, because they were afraid of praying to Judas Iscariot, the traitor! Their fear was so great that Judas Thaddeus became one of the least known apostles.
Eventually the English language made a slight distinction between the two apostles and gave Judas Thaddeus the name Jude. Additionally, prior to this linguistic distinction St. Jude’s intercessory power was revealed to various saints.
For example, according to writer Donald Thorman in his book St. Jude: Saint of the Impossible, “In one vision, Our Lord told St. Bridget to turn to St. Jude with a great deal of confidence, for, said the Lord, ‘In accordance with his surname, “Thaddeus,” the amiable, loving, he will show himself most willing to give help.’ In another vision, Christ commanded the Swedish holy woman to dedicate an altar to St. Jude in her church. ‘The fifth altar,’ He said, ‘must be for Thaddeus who with the purity of his heart will undoubtedly conquer the devil.’”
Read more at Aleteia