“Mara opened the gate” and “The gate was opened by Mara” describe the same basic action but lead with different subjects. Revising the sentence is partly a decision about where the reader’s attention begins.
Follow the paragraph’s subject
If a paragraph tracks Mara’s actions, the active version may carry it naturally forward. If it tracks what happened to the gate, the passive version may connect more smoothly. Read the sentence with its neighbours before applying a blanket rule.
Check the missing actor
“The gate was opened” omits the person responsible. That may be useful if the person is unknown or irrelevant, but it can also conceal information the reader needs. Ask whether the omission is purposeful, accurate and appropriate to the piece.
Try the two versions in an original practice paragraph and compare the emphasis. Do not label one construction weak merely because it belongs to a grammatical category. A useful revision identifies the intended focus, the information that must remain visible and the rhythm that fits the surrounding prose.