It may be said of spiritual exercises as the Apostle speaks of chastening generally, of which indeed they form a component part, that "for the present they are not joyous, but grievous; but afterward they yield the peaceable fruit of righteousness unto them that are exercised thereby." Why the Lord suffers so many of his people to be so long and deeply tried about their interest in Christ, why he does not more speedily and fully manifest his pardoning love to their souls, is a mystery which we cannot fathom. But I have observed that, where the first work was not attended with deep and powerful convictions of sin, it is usually the case, as if what was wanting in depth has to be made up in length, and a slow, continuous work compensates, as it were, for a shorter and more intense one.
I consider it, however, a great mercy where there are these exercises, for I am well convinced that exercise is as much needed for the health of the soul as of the body. Without movement the air becomes pestilential, and water putrescent. Motion is the life of the natural, and equally so of the supernatural, creation; and what are exercises, doubts, and fears, accompanied as they always are by desires and prayers, but means by which the soul is kept alive and healthy? As Hezekiah said, "O Lord, by these things men live, and in all these things is the life of my spirit." But if you cannot see what good exercises have done you, can you not see what evil they have kept you from? They mainly kept you from being entangled in a worldly system; they have preserved you from resting in the form without the power, and kept you from that notional dead-letter faith which has ruined so many thousands. (This extract was taken from a letter to a friend.) Without exercises you could do without a revealed Christ, without manifested pardon of sin, without the love of God being shed abroad in your heart by the Holy Ghost.
And here most are who are not exercised—resting in "a name to live," and in the doctrine without the experience. But, being sick, you need a physician; being guilty, you need mercy; and being a sinner, you need salvation; and all this, not in word and name, but in reality, and divine revelation and application. Your exercises give you errands to the throne of mercy, and make you see in Christ and his precious gospel what otherwise would neither be seen nor cared for. At the same time, it would be wrong to rest in exercises as marks and evidences of grace. Thirst is good as preparatory for water; hunger is good as antecedent to food; but who can rest in thirst or hunger? Without them, water and food are not desired; so, without exercises, Christ, the Water and Bread of life, is not desired nor longed for. But these exercises are meant to quicken longing desires after Christ, and eventually make him very precious.