You Know. 1st Thessalonians 4:2 Apostles were given authority to speak for God before the words of the New Covenant were written. When we read the New Testament, we are reading the words of God's Holy Spirit, put into written form by certain holy men. These are God's commandments to teach and guide us in our thoughts and actions as children of God. For those living in an immoral, promiscuous society, it doesn't take long for behaviors practiced by those who do not know the Lord to seem normal.
Holiness in living is not a common practice today, nor was it in the Greek and Roman empires. In that social setting, the Gospel came to Thessalonica, so not only was the salvation of the soul explained, but also the salvation of a person's life. There is an inward and outward change when someone becomes a child of God. "Old things have passed away; behold, all things have become new." The new nature of the believer is entirely a work of God. The outward change is our responsibility. We have inward strength from the indwelling Holy Spirit, but we also have outward obligations that depend on us.
Instructions given by a teacher are like "commandments," similar to a chain of command. When a superior person provides instructions, it is important to follow them. In this case, these commandments originated with the Lord Jesus and were passed to the believers in Thessalonica through the ministry of the apostle Paul. The will of God is that His people are recognized by their holiness. "Be ye holy, for I am holy." Greeks and Romans were accustomed to immoral living, so these new believers needed to be reminded that Christians are different. The call to live holy lives that please God was made public so that everyone who heard the Gospel and faced what sin looks like in God's eyes knew that the source of truth was the Lord Jesus Christ, who used His servants to write the words spoken publicly. There is unmistakable authority behind what is written in the Scriptures that cannot be ignored or dismissed.