The antonyms for the phrase "was undecided" would be resolute, determined, and certain. Resolute means having a firm decision or purpose while determined means having a strong intention or resolve to do something. Certain, on the other hand, means not having any doubt, unsureness or ambiguity about something. These antonyms reflect a state of assurance and conviction, which is the opposite of being undecided. Being indecisive can lead to missed opportunities, regrets, and wasted time, whereas being resolute, determined, and certain can pave the way forward to achieve success and fulfilment.