Torah teaches us that one’s relationship with God is defined by her relationships with other people – Jews and non-Jews alike.

One of the most fundamental tenets in the Torah is to love your fellow Jew as yourself [Leviticus 19:18]. One of the most frequently stated commandments is ahavat ger (to love the stranger). So it is clear that one’s daily behaviors, personal interactions and expressions of gemilut chasadim (acts of loving kindness) play the greatest role in one’s identity as a good and righteous Jew.

The Chatam Sofer (18th century) teaches:
One may think himself holy if he goes into isolation and practices extreme stringencies and extra religious customs. This is false. For a person to be holy, he must be able to steadily act in a holy manner in all interactions with his fellow man.

This principle is plainly laid out in the Ten Commandments, a majority (six) of which warns of transgressions against others. As Jews, much emphasis is placed on the way we treat one another. Our interpersonal communication, a central theme throughout the Torah, is held to a strict moral code.  Integrity, honesty and kindness are not just virtues, but requirements.

Derech eretz, largely defined as decent, polite, respectful, thoughtful, and civilized behavior, is a recurring theme in the writings of Jewish sages. There are roughly 200 teachings to support the concept; including writings in Pirke Avot (The Ethics of Our Fathers) which stresses the importance of respectful communication.

Chapter Three reads:
All who are pleasing to one's fellow man are pleasing to God and all who are not pleasing to one's fellow man are displeasing to God.

Hillel the Elder is famous for summing up all of the Torah in the following way:
What is hateful to you, do not do to your fellow. The rest is commentary. Go and learn.

Therefore, we are taught that relationships - not knowledge - are the core of Judaism.

Other sources:
Rabbi Jeff Forsythe on Derech Eretz
Derech Eretz in the New Jersey Jewish News