William Guthrie in The Christian’s Great Interest explains that one reason a Christian may experience a lack of assurance is because he has taken no effort in looking into the things of God. Sloth and negligence will cause a lack of assurance.
Indeed, sloth is sin – a serious sin which leads to more and more immediately damaging sins. Thus, one should not expect that by engaging in a continual unrepentant sin (sloth) that such a one should subjectively experience the joy of assurance. Guthrie writes:
The third thing which hindereth in many the knowledge of an interest in Christ is, A spirit of sloth and careless negligence. They complain that they know not whether they be in Christ or not; but as few take pains to be in Him, so few take pains to try if they be in Him. It is a work and business which cannot be done sleeping.
Guthrie proves the point by citing to several verses which exhort believers onto self-examination. One verse will illustrate the point well:
Examine yourselves, to see whether you are in the faith. Test yourselves. Or do you not realize this about yourselves, that Jesus Christ is in you?—unless indeed you fail to meet the test! 2 Corinthians 13:5 (ESV)
The Pulpit commentary explains (in some somewhat antiquated language):
“Examine yourselves.” Self-scrutiny is at once a duty the most urgent and the most neglected. Hence the universal prevalence of self-ignorance. Even men who know a very great deal of the world without are ignorant of the world within, the world of worlds. 1. The momentous point to be tested in self-scrutiny. “Whether ye be in the faith.” Not whether you have faith in you, for all men are more or less credulous, and have some kind of faith in them; but whether you are “in the faith.” The faith here is the gospel, or rather the Christ of the gospel; whether you are in Christ, in the character of Christ. Intellectually and morally, all men are living in the characters of others. The grand thing is to be in the character of Christ, in his principles, sympathies, aims, etc. 2. The momentous conclusion to be reached by self-scrutiny. “Know ye not [emphatic] your own selves, how that Jesus Christ is in you, except ye be reprobates?” If you are in the faith, you are in his character, and he is in your life; nay, your life itself. Should you find you are not in the faith, ye are “reprobates,” counterfeits, spurious, not genuine; tares, not wheat; hypocrites. Here, then, is a work for every man to do—“examine” himself, introspect, scrutinize, decide, and thus know his real moral condition.
Since it is a thing commanded, we are remiss if we neglect to obey. The failure to obey contains the potential for its own correction – a lack of a sense of assurance.
Moreover, when the command is brought to someone’s attention, we have reason to fear for the one who refuses the effort. Before going on a long vacation, we will anxiously double check our passports and luggage. Before buying a house, we will consider the contract. Things which are important to us will command our attention. If one’s state before God is of no interest, then it may just mean that such a person is spiritually dead to the things of God. As Guthrie writes:
Be ashamed, you who spend so much time in reading of romances, in adorning your persons, in hawking and hunting, in consulting the law concerning your outward state in the world, and it may, be in worse things than these;–Be ashamed, you that spend so little time in the search of this, whether ye be an heir of glory or note whether you be in the way that leadeth to heaven, or that way which will land you in darkness forever? You who judge this below you, and unworthy of your pains, any part or minute of your time, it is probable, in God’s account, you have judged yourselves unworthy of everlasting life, so that you shall have no lot with God’s people in this matter.