“There is need of constant watchfulness on the part of the professors of Christianity, lest under the influence of unbelief they depart from the living God, said Dr. John Brown of Edinburgh (1784-1858), commenting on a passage in Hebrews.

“Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin. For we have come to share in Christ, if indeed we hold our original confidence firm to the end.”
Hebrews 3:12-13 esv

Passages such as this ought to arrest a presumptuous believer, and make him immediately more prayerful as he clings more closely to Christ.

Brown continues, There is nothing, I am persuaded, in regard to which professors of Christianity fall into more dangerous practical mistakes than this. They suspect everything sooner than the soundness and firmness of their belief. There are many who are supposing themselves believers who have no true faith at all — and so it would be proved, were the hour of trial, which is perhaps nearer than they are aware, to arrive. And almost all who have faith suppose they have it in greater measure than they really have it. There is no prayer that a Christian needs more presently to present than, “Lord, increase my faith” and “deliver me from an evil heart of unbelief.” All apostasy from God, whether partial or total, originates in unbelief. To have his faith increased — to have more extended, and accurate, and impressive views of ‘the truth as it is in Jesus’ — ought to be the object of the Christians most earnest desire and unremitting exertion.