1-3: Paul introduced himself as an apostle entrusted by God with the purpose of serving the elect by sharing the Gospel and teaching the truths of God.
4: The letter is to Titus, who Paul led to the Lord. Titus was left on the island of Crete to plant organized churches for the believers that lived there.
5-9: Church leaders should be chosen on the basis of their moral character. They must also hold firm to God's word to give accurate instruction and rebuke false teaching such as taught by the Judaizers in Crete.
10-14: The Judaizers were a "Christian" sect who heretically added to the Gospel by teaching that ceremonial laws such as circumcision had to be observed in order to be a Christian.
15-16: Those teaching circumcision were false converts. Although they claimed to know God, their works were proof they didn't.
1-10: Older men and women in the church should be models for younger Christians by living godly, sober, self controlled lives.
11-14: The Gospel is our reason for doing good and obeying God. Jesus sacrificed himself to purify us from sin and redeem us from eternal punishment. He redeemed us for his own possession and is training us to live godly, self controlled lives through the Bible and the Holy Spirit.
15: Titus was commanded to declare the contents of this letter authoritatively to the church in Crete.
1: Christians should obey government authorities in all areas that do not contradict God's law, which is the higher authority. For example, if the CEO of a company creates a rule for employees and your manager tells you to violate it, you must obey the CEO who's authority overrides the manager's. The same is true of God's law vs government laws. We should also be on the lookout for opportunities to do good works and seek ways to carry them out.
2: The high standard of loving neighbor as self plays out in commands like these.
3-8: We should treat others the way verse 2 commands in light of the fact that we too were vile sinners mercifully saved through Jesus. He cleansed us of our sin and justified us so we could obtain eternal life.
9-11: Christians are to avoid unnecessary, foolish controversies because they cause division in the body of Christ. This is likely referring to overemphasis on secondary, inconsequential, biblical issues as well as controversy in personal matters. After the third warning have nothing to do with the person causing unnecessary division.
12-15: Paul closes with instruction for Titus to come to Paul in Nicopolis where he was spending the winter. Artemas or Tychicus would be Titus' replacement in Crete.