Final answer:
If preaching and faith are considered vain, the consequence would be a loss of integrity and credibility, leading to public disgrace or divine retribution, as echoed in historical sermons and religious writings discussing the authenticity of religious practices.
Step-by-step explanation:
The question suggests a context of theological reflection and is reminiscent of religious sermons or discussions touching on matters of faith and doctrine. When the statement suggests that if preaching and faith are vain, then those engaged in them are found to be false or deceptive, it reflects a concern with the authenticity and integrity of religious practice and belief. Such a scenario implies that if the foundational beliefs or teachings are without substance or truth, the consequence would be a loss of credibility and respect among believers and the wider community. This is illustrated by historical sermons and writings, which often warned religious communities that insincerity and failure in their spiritual undertakings would lead to public disgrace or divine retribution.