Devote yourselves to prayer, being watchful and thankful. And pray for us, too, that God may open a door for our message, so that we may proclaim the mystery of Christ, for which I am in chains. Pray that I may proclaim it clearly, as I should. Be wise in the way you act toward outsiders; make the most of every opportunity. 
—Colossians 4:2–5

Verse–by–Verse Insights from Dr. John MacArthur

Verse 4:2
Devote yourselves. The Greek word for “devote” means “to be courageously persistent” or “to hold fast and not let go” and refers here to persistent prayer (See also, Acts 1:14; Romans 12:12; Ephesians 6:18; 1 Thessalonians 5:17; and Luke 11:5–10 and 18:1–8.)

Being watchful. In its most general sense this means to stay awake while praying. But Paul has in mind the broader implication of staying alert for specific needs about which to pray, rather than being vague and unfocused. (See also Matthew 26:41; Mark 14:38; Luke 21:36.)

Verse 4:3
A door. An opportunity (See also, 1 Corinthians 16:8,9; 2 Corinthians 2:12.)

The mystery of Christ. This refers to truth, hidden until now, but revealed for the first time to the saints in the New Testament. Such truth includes the mystery of the incarnate God (Colossians 2:2, 3, 9), Israel’s unbelief (Romans 11:25), lawlessness (2 Thessalonians 2:7), the unity of Jew and Gentile made one in the church (Ephesians 3:3–6), and the rapture of the church (1 Corinthians 15:51).

Verse 4:5
Outsiders. This refers to unbelievers. Believers are called to so live that they establish the credibility of the Christian faith and that they make the most of every evangelistic opportunity.

What opportunities has God given you recently to serve someone?

Notes taken from the verse–by–verse notes in The MacArthur Study Bible.