This paper presents a unified structural theory for first-order logic, focusing on the invariant duality between proof and truth. We investigate how syntactic proofs and semantic models, traditionally distinct yet deeply interconnected through theorems like Gödel's completeness theorem, can be understood within a single coherent framework that reveals their inherent structural correspondence. The theory formalizes the notion of invariance, demonstrating how the fundamental relationship between provability and satisfiability persists across various structural transformations of logical systems. We introduce categorical constructions that encapsulate first-order theories and their models, establishing a functorial duality that rigorously connects proof-theoretic structures to model-theoretic ones. This approach not only re-affirms classical results of soundness and completeness as natural consequences of a deeper structural isomorphism but also provides novel insights into the robustness and universality of logical validity. The work aims to offer a foundational perspective that enriches the understanding of the nature of logical reasoning and truth preservation.