What does it Mean to Repent and Believe?

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Now after John was arrested, Jesus came to Galilee proclaiming the good news of God, saying, “The time is fulfilled, and the Kingdom of God has come near; repent and believe in the good news.”
— Mark 1:14–15

What is it that the writer of Mark meant by the words “repent” and “believe?”

Looking at the word translated from the Greek as “repent” we find the word is…