What is the convincing response to one who abandons prayer, claiming that his heart is full of faith?

A person who abandons prayer cannot have a heart filled with faith; anyone who claims otherwise is a liar. The Prophet, peace and blessings be upon him, said: "The covenant between us and them is prayer; whoever abandons it has disbelieved." The flourishing of the heart with faith must manifest its effect upon the limbs, and prayer is the greatest of the actions performed by the limbs. It is impossible for a man to have firm faith in his heart that Allah has enjoined prayer upon him, yet live his entire life without prostrating to Allah even once. This can only stem from hypocrisy and apostasy in the heart, not from true faith.