Perhaps this book has flown under your radar. I just read it for the first time last month shortly after learning about it, and I’m surprised that I don’t recall hearing others recommend it before.

Donald S. Whitney. How Can I Be Sure I’m a Christian? What the Bible Says about Assurance of Salvation. Colorado Springs, CO: NavPress, 1994. 153 pp.

Here’s an extended outline of the book (not including the application questions at the end of each chapter):

Chapter 1. Assurance of Salvation–Is It Possible?

• It is possible, indeed normal, for the Christian to experience assurance of salvation.
• It is possible, indeed normal, for a non-Christian to have a false assurance of salvation.

Chapter 2. Having Doubts about Your Salvation

It is possible, indeed normal, for Christians to have occasional doubts about their salvation.

• Doubting assurance is not unbelief.
• The causes of doubt are many.
  • Spiritual immaturity may contribute to doubts about assurance.
  • Sensitivity to sin may cause confusion about assurance.
  • Comparison with other Christians may cloud assurance.
  • Childhood conversion affects the assurance of some.

Chapter 3. The Basis of Assurance

The assurance of salvation rests primarily on the character of God, the works of Jesus Christ, and the truth of God’s promises.

Chapter 4. An Inner Confirmation

• Assurance may be experienced partly through the inner confirmation of the Holy Spirit.
• How does the Holy Spirit give Christians this assurance?
  • He opens our minds to understand the Bible in ways that give us assurance.
  • He guides our thinking about the biblical marks of salvation in our lives.
  • He brings Scripture and its truths to our minds in various ways that assure us.
  • He causes an inner sense of assurance without words.

Chapter 5. Signs of Eternal Life

Assurance may be experienced partly through the presence of the attitudes and actions the Bible says will accompany salvation [1 John].

• Do you share the intimacies of the Christian life with other believers?
• Do you have a deep awareness of your sin against the word and love of God?
• Do you live in conscious obedience to the word of God?
• Do you despise the world and its ways?
• Do you long for the return of Jesus Christ and to be made like him?
• Do you habitually do what is right more and sin less?
• Do you love other Christians sacrificially and want to be with them?
• Do you discern the presence of the Holy Spirit within you?
• Do you enjoy listening to the doctrines the apostles of Jesus taught?
• Do you believe what the Bible teaches about Jesus Christ?

Chapter 6. A Spiritual Mind-set

• Only those who are spiritually minded are Christians.
• You are spiritually minded when you think about the things of God spontaneously and without external causes.
• You are spiritually minded when you think about the things of God more than anything else.
• You are spiritually minded when you think about the things of God with more delight and enjoyment than anything else.
• You are not spiritually minded if “God is not in all [your] thoughts.”

Chapter 7. Things That Erode Our Assurance

A true Christian may lose a sense of assurance of salvation because . . .

• he or she refuses to deal with known sin.
• of spiritual laziness.
• of satanic attacks.
• of trials or harsh circumstances.
• of illness or temperament. [Cf. Jon Bloom, “When You’re in a Spiritual Storm, Trust Your Instruments.”]
• God seems to withdraw a sense of his presence and blessing.

Chapter 8. Common Problems with Uncertainty

• Those converted as children may experience special difficulties with assurance.
  • Those who remember little else besides following Christ sometimes have doubts that those with adult or dramatic conversions do not.
  • Concrete childhood thinking differs from more abstract adult thinking.
  • An awareness of the Lordship of Christ must expand to cover all the ever-expanding circle of life that comes with maturity.
• Stay-at-home mothers of young children may experience special difficulties with assurance.
• True assurance won’t lead to spiritual carelessness.
• Those worried about the unforgivable sin have not committed it.

Chapter 9. False Assurance of Salvation

• Sources of a false assurance of salvation
  • A public commitment or outward response to the gospel
  • Baptism
  • Involvement with church
  • A strong Christian family heritage
  • An abundance of good deeds
  • An extraordinary experience
  • A dramatic personal or lifestyle change
  • Material blessing and financial security
  • A false understanding of God
  • A false understanding of sin and hell
• Characteristics of the falsely assured
  • They are either unconcerned or angry when warned about false assurance.
  • They are either legalistic or loose with spiritual disciplines and duties.
  • They are either very weak in or very confident of their Bible knowledge.
  • They have either a vicarious Christianity or an overly independent spirit.
  • They may be constantly resisting the truth or never able to come to the truth.

Chapter 10. What to Do If You’re Still Not Sure

• Don’t take for granted that you understand the gospel.
• Think deeply about the gospel.
• Repent of all known sin.
• Submit everything to the Lordship of Christ.
• Meditate much on 1 John.
• Don’t doubt the promises of God.
• Believe as best you can and pray for greater faith.
• Practice the spiritual disciplines.
• If you really love God, take assurance because non-Christians don’t love God passionately.
• If you hate your sin, take assurance because non-Christians don’t hate sin deeply.
• If you’ve never been baptized, present yourself as a candidate in obedience to Christ.
• Don’t neglect the Lord’s Supper.
• Don’t compare earthly fathers to your Heavenly Father.
• Seek godly counsel if the doubts persist.
• Pray for assurance.
• Wait patiently upon God to give you a fuller experience of assurance.

Related: Don Carson on Assurance of Salvation