Why was Petrarch the first humanist?

Because Petrarch was the earliest representative of studying classical culture from the perspective of humanism during the Renaissance. It is proposed to replace "God's Thought" with "Man's Thought"

He widely collected ancient manuscripts of Greece and Rome, dared to break through the theological concept of the Middle Ages, and put people and real life at the center to interpret classic works with the vision of the new era. His study of classical culture had an impact on the European Renaissance and his own creation.

Petrarch wrote many poems and essays in Latin. These works praise the nobility and wisdom of human beings, advocate that people can pursue secular happiness and the right to honor, and challenge the theocracy and asceticism advocated in the Middle Ages. He also believes that a person's nobility lies not in birth, but in behavior.