As a thief in the night. The day of the Lord comes unexpectedly, without warning to those unprepared. The same image Jesus used (Matthew 24:43) and Peter (2 Peter 3:10). No one can calculate its hour.

The unexpectedness is a warning to the unprepared and a call to readiness for all. The believer cannot know the day, so he must always be ready. Date-setting contradicts the very nature of the coming.

Children of light... children of the day. The believer's identity. He belongs to the light, to the day — not to the darkness that will be surprised by the thief. The coming that terrifies the world is the homecoming the believer awaits.

Because the believer belongs to the day, the day of the Lord holds no terror for him. He is not in darkness that the day should overtake him as a thief (verse 4). The same event is dread to the children of darkness and hope to the children of light.

God hath not appointed us to wrath, but to obtain salvation. The believer's destiny is settled — not wrath, but salvation. The wrath to come (1:10) is not for those who are in Christ.

A great assurance. The day of the Lord brings wrath on the world but salvation to the believer. The same God who appoints the wicked to judgment has appointed His people to salvation through Christ. The outcome is not in doubt.

The shortest verse in the New Testament Greek text — two words. Rejoice evermore. A command to constant joy, possible only because the joy is in the Lord (Philippians 4:4), not in circumstances.

Joy is commanded, which means it is in some measure a matter of the will, not merely of mood. The believer can choose to rejoice in the Lord even when feelings lag, because the grounds of joy — Christ and His finished work — never change.

Pray without ceasing. Not unbroken verbal prayer, but a constant attitude of prayer — a life lived in continual communion and dependence, ready to turn to God at any moment.

The believer's life is to be saturated with prayer, like a continual conversation running beneath all activity. The incense altar of the tabernacle burned continually (Exodus 30:8); so the believer's prayer ascends without ceasing.

In every thing give thanks. Not for everything, but in everything — in every circumstance, thanksgiving is possible and commanded, because God is at work in all things for good (Romans 8:28).

This is the will of God. Alongside sanctification (4:3), here is another clear statement of God's will — a thankful heart in all circumstances. When believers wonder what God wants, here it is: rejoice, pray, give thanks.

Prove all things — test, examine, discern. The believer is not to accept teaching uncritically nor reject it unthinkingly, but to test everything against the truth and hold fast that which is good.

A call to discernment. The Bereans were commended for this (Acts 17:11), examining the Scriptures to test even Paul's preaching. The believer keeps what passes the test and discards what fails. Neither gullible nor cynical, but discerning.

Sanctify you wholly... spirit and soul and body. Sanctification is to extend to the whole person — every part. The believer is not sanctified in compartments but entirely.

Preserved blameless unto the coming of our Lord. The next verse gives the ground of confidence — Faithful is he that calleth you, who also will do it. The God who began the work will complete it (Philippians 1:6). Sanctification is His work, guaranteed to completion.