There are many synonyms for the phrase "was convinced." Some of these include "was certain," "was persuaded," "was satisfied," "was sure," "was confident," "was assured," and "was convinced beyond a doubt." These various phrases all convey the idea that someone had a firm belief or conviction about something. Using synonyms can help to add variety and depth to your writing. By choosing a different phrase to express the same idea, you can avoid repetition and make your writing more engaging and interesting to read.