Final answer:
The phrase 'bestowed upon Paul in vain' refers to the grace or a calling that Paul received, which he valued and used effectively, impacting others like Thecla.
Step-by-step explanation:
The phrase 'what was not bestowed upon Paul in vain' likely refers to the grace of God or a specific calling or responsibility that Paul received, which he did not take lightly or for vanity. In the texts provided, we see references to Paul's influence and the response of Thecla, who found great value and conviction through Paul's teachings. This indicates that whatever was bestowed upon Paul, it was indeed fruitful and significant in the lives of others, hence not in vain.