Final answer:
The missing word in the provided phrase is 'pray,' which complements the biblical theme of staying vigilant against temptation and acknowledging the frailty of the flesh.
Step-by-step explanation:
The missing word in the phrase "Watch ye and ___________, lest ye enter into temptation. The spirit truly is ready, but the flesh is weak" is "pray." This phrase is a biblical reference, depicting the constant vigilance and spiritual discipline required to resist temptation. The contrast between the willing spirit and the weak flesh captures the human struggle between moral aspirations and physical desires.