We prove a new convergence condition for the activity expansion of correlation functions in equilibrium statistical mechanics with possibly negative pair potentials. For non-negative pair potentials, the criterion is an if and only if condition. The condition is formulated with a sign-flipped Kirkwood-Salsburg operator and known conditions such as Koteck${ý}$-Preiss and Fern${á}$ndez-Procacci are easily recovered. In addition, we deduce new sufficient convergence conditions for hard-core systems in $\mathbb R^d$ and $\mathbb Z^d$ as well as for abstract polymer systems. The latter improves on the Fern${á}$ndez-Procacci criterion.