Learn Spanish (Latin America): Lo que los creyentes vieron fue un milagro.

Qué with an accent is used for asking questions. When you use 'what' in a statement to mean 'that which' or 'the thing that', you need lo que. Here, it translates literally to 'The thing that the believers saw'.

Since we are learning about verbs of change, could I use llegó a ser instead of fue?

You could, but it changes the story! Fue un milagro simply states a fact: what they saw was a miracle. Using the culmination verb llegó a ser (came to be) would imply a long, difficult process, suggesting the event eventually turned into a miracle over time.

Why use the preterite vieron instead of the imperfect veían?

Vieron is used because seeing the miracle was a specific, completed event. If you used veían (were seeing / used to see), it would describe an ongoing background action without a clear end, like 'what the believers were looking at [when something else occurred].'

How does the word creyentes (believers) get its spelling?