J.C. Ryle, from A Call to Prayer:

I do not deny that a person may pray without heart and without sincerity. I do not for a moment pretend to say that the mere fact of a people praying, proves everything about their soul. As in every other part of religion, so also in this — there may be deception and hypocrisy.

But this I do say — that not praying is a clear proof that a person is not yet a true Christian. They cannot really feel their sins. They cannot love God. They cannot feel themselves a debtor to Christ. They cannot long after holiness. They cannot desire Heaven. They have yet to be born again. They have yet to be made a new creature. They may boast confidently of election, grace, faith, hope and knowledge, and deceive ignorant people. But you may rest assured it is all vain talk — if they do not pray.