Final answer:
Based on the historical context of the passage, if our preaching and faith are in vain, we are considered 'false witnesses', due to the breach of trust and failure in integrity.
Step-by-step explanation:
If our preaching and faith are considered vain, according to the quoted text, we would be found to be false witnesses. This conclusion is drawn from the historical context provided in the passage, which suggests that living a life of falsehood, especially in a spiritual or communal sense, leads to being 'made a story and a by-word throughout the world'. Here, 'dealing falsely with our God' suggests a breach of trust or faith, which aligns with the role of a false witness rather than just being liars, sinners, or fools on a general level. The emphasis in this context is on the public and collective consequences of such actions, accentuating the need for integrity and truthfulness in both preaching and living out one's faith.