Final answer:
The principle at the heart of Jesus's teachings in the Gospels is the command to love one's neighbor as oneself, emphasizing compassion and spiritual equality, which underpins the message of Christianity founded on his life and teachings.
Step-by-step explanation:
The principle that encapsulates the message of Jesus, as described in the Gospels, is the commandment to love one's neighbor as oneself. This teaching is a cornerstone of Christian ethical teaching and reflects the radical universalism that Jesus promoted, which included kindness and compassion to all, irrespective of social status. According to the Gospels, Jesus began preaching this message of salvation at the age of thirty, spreading the idea that mankind could be saved through seeking forgiveness from God. His life and teachings, including his death and the claim of resurrection, are documented in the New Testament by his disciples turned apostles, which led to the birth of Christianity. The message of Jesus reached various social classes and was revolutionary against the Roman hierarchical system, emphasizing the spiritual equality of all human beings.