Police said the threat was made by a former employee of the school and, although it was credible, it was not directed at students. Police now consider the case closed.
The threat had prompted the campus to close this week, but school officials said parts of the campus would reopen to students and staff as soon as Thursday.
On Friday, students will have the option to meet with teachers or counselors if needed. There will be additional security next week when students return for exams.
School officials learned about the threat Tuesday and sent a message to parents about the threat that afternoon.
"Today, we learned of a credible threat to the Mater Dei campus and contacted the Santa Ana Police Department," said the note from Mater Dei President Michael Brennan and Principal Frances Clare.
"Safety and sweep protocols were implemented," the officials said.
"We acknowledge the alarming nature of this message; however, we at Mater Dei take all threats made to the school seriously."
Police did not identify the former employee or disclose details about the threat and who it was addressed to or how it was sent. Further details about the threat, how it was sent and whether it was aimed at anyone in particular were not disclosed.
City News Service contributed to this report.