I Thessalonians 1:4, 5

"Knowing, beloved brethren, your election by God." (I Thessalonians 1:4)

Brethren is a common word in this letter. Paul feels a close kinship to other believers. They were like brothers and sisters.
God had chosen (elected) these believers. "Election" emphasizes God's purpose for saving us. He has chosen us for a purpose: to worship Him and to minister to others in His name. Paul knew the mission for which God had chosen these believers. In the last half of this chapter, Paul will describe their ministry.

"For our Gospel did not come to you in word only, but also in power, and in the Holy Spirit and in much assurance, as you know what kind of men we were among you for your sake." (I Thessalonians 1:5)

The good news (Gospel) was what Paul and the other Apostles preached. Paul preached the Gospel using words, but man's alone do not save. The Gospel is the power of God to salvation. The Gospel has a divine power. No one is saved unless the Spirit draws him or her to God. Those who are not yet Christian will always move away from God unless the Spirit works on them.
In the original Greek, there is no "in" before much assurance. This links the Holy Spirit and much assurance. He gives people a divine assurance that the Gospel is true.
The messenger is part of the message! Many have refused the Gospel because of the way Christians act. Paul lived a life that caused people to believe his message. His behavior helped prove his words.