The antonyms for the phrase "are source strength" are "not a source of weakness." When something is not a source of weakness, it means that it does not weaken or undermine a system or individual. This could refer to a person's physical or emotional strength, or the strength of a company or organization. In other words, something that is not a source of weakness is a positive influence, providing support and stability instead of creating problems. At times, it could also refer to something that does not take away from the strength of something else, but rather enhances or complements it.