The antonyms for the phrase "are a source of strength" can be "are a cause of weakness" or "are a hindrance to growth." In life, there are situations when the people who we consider as our strength, might become a source of tension, negativity or pull us down. They could be people who constantly complain or discourage us, or situations that demand a lot of energy and attention. These things can weaken our resolve and make us feel drained instead of empowered. Therefore, it's essential to evaluate and re-evaluate our relationships and situations to ensure that they remain a source of strength and positivity.