Final answer:
The Supreme Court likely used the Freedom of Speech Clause from the First Amendment to justify its decision in Christian Legal Society v. Martinez, which involved issues of expressive association and free speech in the context of a university's student organization policy.
Step-by-step explanation:
In the case of Christian Legal Society v. Martinez, the Supreme Court most likely used the Freedom of Speech Clause from the First Amendment to justify its reasoning. The Christian Legal Society challenged the University of California Hastings College of the Law's policy requiring registered student organizations to allow any student to participate, be a member, or seek leadership positions in the organization, regardless of their status or beliefs. The case centered on whether the school's policy infringed the society's free speech, expressive association, and free exercise of religion rights because the society wanted to exclude individuals who did not share their beliefs.
The court held that the law school's condition on recognizing student groups was a reasonable, viewpoint-neutral condition on access to a limited public forum, thus satisfying constitutional standards. While the society also argued under the Free Exercise Clause, the decision primarily addressed issues of expressive association and free speech, rather than the establishment of religion, making the Freedom of Speech Clause more central to the Court's justification.