Byzantine Fault Tolerant (BFT) consensus exhibits higher throughput in comparison to Proof of Work (PoW) in blockchains. However, BFT-based protocols suffer from scalability problems with respect to the number of replicas in the network due to their inherent quadratic message complexity. Previously, proposed solutions improve BFT performance for normal operation, but will fall back to quadratic message complexity once the protocol observes a certain number of failures. To address this issue we propose Proteus, a new BFT-based consensus protocol which elects a subset of nodes c as a root committee. Proteus guarantees stable performance, regardless of the number of failures in the network and it improves on the quadratic message complexity of typical BFT-based protocols to O(cn) messages, where c << n, for large number of nodes n. We tested our protocol on 200 Amazon EC2 instances, with two different baseline BFT protocols (PBFT and Bchain) for comparison. In these tests, our protocol outperformed the baselines by more than 2× in terms of throughput as well as latency.