What is the ruling on using the phrase "weakness of religion"?

There is no harm in this phrase. Its intent is to describe the weakness of a person's religiosity and uprightness, not the weakness of the religion itself. Faith increases and decreases, and strengthens and weakens, as is the belief of the Ahl al-Sunnah wal-Jama'ah (Sunni Muslims). Therefore, there is no harm in describing a person as "weak in faith."