Must the prayer be repeated if the worshiper becomes certain of the presence of impurity on his garment after completing the prayer, knowing that he had doubted its presence before the prayer and considered it a mere delusion?

What you did by ignoring the doubt about the impurity of the garment and performing congregational prayer is correct, and your prayer in a garment about whose impurity you have doubt is valid, because the default state is its purity. If you find impurity after the prayer, and it is probable that it occurred after it, then it is attributed to the time after the prayer. If you are certain that you prayed in an impure garment, your prayer is valid due to your ignorance of the impurity.