The answer is: Thou shalt not commit adultery

Exo 20:14

In the context of the Exodus narrative, the seventh commandment, "Thou shalt not commit adultery," stands as an essential principle for fostering healthy relationships. It not only serves to maintain trust within marriages but also highlights the importance of fidelity as a cornerstone for social bonds in ancient Israelite society. Adultery was more than just infidelity; it was perceived as a crime against both the spouse and the community, leading to an unwinding of social fabric.

Interestingly, this commandment aligns with various ancient law codes, such as the Code of Hammurabi, which also emphasized family integrity and punishing infidelity. This reflects a broader cultural recognition of the vital full-hearted compliance necessary for social stability.

Moreover, the significance of this commandment echoes throughout the Bible. The prophetic texts—especially in books like Hosea—metaphorically frame Israel's relationship with God as a marriage, illustrating infidelity through idolatry. When the people stray from divine commitment, it's this deep betrayal biblically echoed back to the concept of marital unfaithfulness.

Also noteworthy is the response of Jesus in the New Testament, where he expands this commandment to the heart's desires, indicating that even thinking with a fleeting intention to trespass can carry moral weight. Thus, this commandment elegantly encompasses not just actions but thoughts and intentions, advocating for a deeper, holistic approach to fidelity in all human interactions.