Commenting on John 14:1, J. C. Ryle insightfully wrote:

“Heart-trouble is the commonest thing in the world. No rank, or class, or condition is exempt from it. No bars, or bolts, or locks can keep it out. Partly from inward causes and partly from outward,—partly from the body and partly from the mind,—partly from what we love and partly from what we fear, the journey of life is full of trouble. Even the best of Christians have many bitter cups to drink between grace and glory. Even the holiest saints find the world a vale of tears.”

The English Puritan theologian, Richard Sibbes, observes from these words of Christ, “that the best Christians are subject to be troubled, to be pensive, and dejected more than should be.” At some length, he ministers to the heart of the suffering saint and tenderly serves the biblical nourishment of Christ-centered comfort. In answer to the natural voice of the troubled heart, Sibbes gently reminds us that such experiences are “partly for conformity to our Head [Christ], and partly that we may be known to ourselves; that we may discern where our weakness lieth, and so be better instructed to seek to Him in whom our strength lieth.”

He then offers some guidance for diagnosing the troubled heart. Indeed, not all troubled hearts reflect a holy agitation of soul. Sometimes we may be downcast, deeply troubled, fearful, and captive to gloom in the heart. The point is that there is (a) a holy kind of troubled heart, as demonstrated by Christ Himself (John 11:33; 12:27; 13:21), and (b) there  is an unholy kind of troubled heart, as indicated by Christ’s admonition to “stop letting your heart be troubled” (force of the present active imperative) in John 14:1 (cf. John 14:27). He would not instruct them to stop letting their hearts be troubled if it were not unholy and unhealthy. To help diagnose the troubled heart, Sibbes writes:

“But how shall we know that our hearts are more troubled than they should be? For I lay this for a ground: That we may sin in being over much troubled at things for which it is a sin not to be troubled. If they had not been at all affected with the absence of Christ, it had been a sin, and no less than stupidity; yet it was their sin to be over much troubled. In a word, therefore, for answer, a trouble is sinful when it hinders us in duty or from duty; when it hinders us in duties to God or to others; or from duty, that is, when the soul is disturbed by it, and, like an instrument out of tune, made fit for nothing, or like a limb out of joint, that moves not only uncomelily, but painfully, and becomes unfit for action. When we find this in our trouble, we may know it is not as it should be.
… Satan loves to fish in these troubled waters.”

A troubled heart is a given in this life. But in an exceedingly deep cause for trouble, Christ said: “Let not your hearts be troubled. Believe in God; believe also in me” (John 14:1). This exhortation reminds those who trust in Christ that we do not have to go through life mourning all the way to heaven. Faith is the language that communicates these realities to the human heart; Christ is the cure: “Trust in God, trust also in Christ.”