Let $T$ be a universal (universally axiomatizable) theory of signature $\mathcal L$ without functional symbols. Let also $M$ to be an $\mathcal L$-structure. Prove that if every finite substructure of $M$ is a model of $T$, then $M$ is also a model of $T$.
I need a hint on how to approach this problem because I can't figure out where to start. Thanks!