Charles H. Spurgeon on Psalms 99:6

His priests. At the foundation of this there is another spiritual idiom, that, namely, according to which all are called priests who possess what constitutes the essence of the ordinary priestly office (although not the externals), inward connection with God, free access to the throne of grace, and the gift and power of intercessory prayer. This figurative idiom occurs even in the law itself, compare Ex 19:6, where it is said to all Israel, "Ye shall be unto me a kingdom of priests, and an holy nation." F.W. Hengstenberg.