Irenaeus of Lyons

Most early Christians had vague theology. Irenaeus had a system. He's the first Father to lay out a comprehensive answer to 'what does Christianity actually teach' — against Marcion, against the Gnostics, against the spiritualised Christ-as-pure-idea heresies that were everywhere in his time. He invented apostolic-succession-as-argument: not as a power claim, but as a fact-checking tool. If your teacher's teacher's teacher didn't say it, it isn't apostolic. Every later orthodox theologian inherits his playbook.