Regardless of what $X$ is, "$P(x)$ holds for all but finitely many $x\in X$" means that the set $\\{x\in X:\
eg P(x)\\}$ is finite. If $X$ is finite, then $\\{x\in X:\
eg P(x)\\}$ is always finite, since it is a subset of $X$. So if $X$ is finite, every property holds for all but finitely many $x\in X$.