Why was Galileo Galilei placed under house arrest?

Galileo Galilei was placed under house arrest following his 1633 trial by the Inquisition, where he was found "vehemently suspect of heresy" for his support of the Copernican theory.12

The conflict stemmed from Galileo’s publication of the Dialogue Concerning the Two Chief World Systems, Ptolemaic & Copernican in 1632. Although he had received permission from Pope Urban VIII to write about Copernican theories, he was instructed to treat them only hypothetically. A special commission later concluded that Galileo had failed to do so and had instead actively defended the theory, which the church had previously declared heretical in 1616.13

During the trial, Galileo was confronted with a 1616 edict that forbade him from holding, teaching, or defending the Copernican view. To resolve the case, he confessed to overstating his position and was formally required to abjure his beliefs. Consequently, he was sentenced to life imprisonment, which he served as house arrest until his death in 1642.124