The truth vs. fear
The play shows us that truth can be a dangerous weapon, an instrument for changing the world, even when the authorities want things to stay the same. Galileo believes that everyone wants to know the truth as much as he does, but this isn't the case, as many oppressive governmental and religious forces are trying to maintain order by amplifying the status quo over empirical scientific progress. The truth he wants other people to see and accept threatens the traditional way of life and promises to cause social unrest. Thus, even though people know that Galileo's discoveries are true and correct, they are kept silent by their own fear.
Sense of Self and Loyalty to Others
The play is very concerned with strength of character: in terms of both believing in oneself and staying true to others. Galileo continues to believe in his own ideas throughout the play, even as authorities tell him that he must stop making progress in his field of choice. Thus, the narrative concerns a man who is loyal to his intellect over everything else, even the marriageability of his daughter and his own health.
This story is also about the loyalty that people feel to one another. Andrea worships Galileo and believes in his work wholeheartedly, becoming the thinker's protege. All of Galileo's pupils and followers are loyal to him, even in the face of potential oppression from the church. It is not until Galileo recants on his findings that Andrea abandons him, so betrayed does he feel by Galileo's unwillingness to remain firm in his beliefs and research.
Religion
The play depicts the complexity of the Roman Catholic Church. It has a monopoly on the Italian government and influences policy as well as the lives of ordinary people. In particular the Inquisition, using the name of God as an excuse for tortures and murdering, tries to keep power in the church's hands and out of the hands of the people. Religion, as an institution, is portrayed as a corrupt force in Italy, a system of governance that takes the power and autonomy away from the people and forces them to accept a more deterministic fate.
While religious institutions are depicted as oppressive and selectively enlightened in the play, Galileo is not actively anti-religious. For him, the pursuit of science is a religious act in itself. By investigating how the world works, he seeks to understand the mysteries of the universe: not so he can believe less in God, but so that he can become more enlightened. His work is misinterpreted by his contemporaries as heretical and anti-religious because of their social implications, but his devotion to science, to doubt, and to proof, is all part of his own particular relation to spiritual existence.
Parenthood
Galileo has a daughter, whose life depends on her father’s actions. Additionally, his young helper, Andrea, has a mother, Mrs. Sarti, who also works for Galileo. A reader sees an example of two different parents, who love their children in different ways. While Mrs. Sarti values her son more than anything, Galileo loves truth more than he cares about the well-being of his daughter, sacrificing her chance at a profitable marriage in order to continue to spout his controversial ideas. While the play does not address this theme directly, parenthood and family often come into play in the narrative.
Class Unrest
Galileo's findings are not simply controversial because they disrupt what has been previously believed, but because they promise to complete disorient and disrupt the economic and political structures in Italy. Religion, as an institution, works as a kind of pacifying mechanism for the impoverished. The little monk describes the plight of his parents, working-class people, whose only comfort from the hardship of their lives is the conviction that they are fulfilling God's will. By positioning God at the head of their existence, these working people are able to embrace their difficult lives as part of their fate, determined by an entity other than themselves. The knowledge that the universe does not work the way that the Bible says it does threatens the very fabric of this social contract. If working-class people begin to understand that their lots in life are determined by governmental organizations, not an all-powerful deity, they will begin to resist their working conditions, realizing that the social hierarchy in place is completely arbitrary. Ludovico describes this social unrest when he tells Galileo that if the workers on his vineyard got wind of the Copernican model of the universe, they would forgo their labors in favor of gossip. Thus, the threat posed by Galileo's discoveries are not simply intellectual, but political, in that they go against everything holding together the hierarchical system at play in Italy at that time.
Corruption
Several times in the play, Galileo says, "Someone who doesn't know the truth is just thick-headed. But someone who does know it and calls it a lie is a crook." In his estimation, it is one thing not to know the truth, to be ignorant, but it is quite a different thing to know the truth and yet deny it. This signals that someone is a "crook," completely unethical, if they are ignoring the truth that is right in front of their eyes.
Martyrdom
The climactic moment in the play is the moment that Galileo decides to recant, taking back all his ideas to avoid punishment in the Inquisition. Rather than become a martyr for his ideas, a choice that would likely grant more dignity to his ideas and embolden others, he chooses his life. Galileo makes a huge sacrifice in that he must be imprisoned by the Inquisition for the rest of his life, forced to pursue his studies secretly and supplicate to authorities he does not respect, but at least he gets to live. The reason why his followers, especially Andrea, are so disappointed in him, is that he does not choose to become a martyr and imbue his findings with more meaning.
