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Craig's theorem says that a theory with a computably enumerable axiomatization has an equivalent decidable axiomatization. The Craig trick gives a constructive proof by encoding an enumeration index in repeated conjunctions of the enumerated axiom.

The Craig trick replaces a computably enumerable axiom set by an axiom set that is a computable set with the same consequences. If the original set is nonempty, arrange a total computable sequence $A_1,A_2,\dots$ listing it with repetitions. This is possible even for a finite axiom set: once an axiom has appeared, output that axiom again at stages with no new output. An empty axiom set is already decidable.

Replace $A_i$ by the fully parenthesized conjunction $B_i$ of $i$ copies of $A_i$, using a fixed association convention. Each $B_i$ is logically equivalent to $A_i$. To test a candidate string of length $m$, compute only $A_1,\dots ,A_m$, construct the corresponding $B_i$, and compare strings. A conjunction of $i$ nonempty formulas has length at least $i$, so no later $B_i$ can equal the candidate. This finite computation decides membership.

The explicit total enumeration matters: waiting for the first $m$ distinct outputs of an arbitrary enumerator could run forever when it enumerates a finite set. The construction proves Craig's theorem.