"He called you to this through our gospel, that you might share in the glory of our Lord Jesus Christ." NIV translation
God called the Thessalonian believers to Christ through the sharing of the good news (that's what gospel means) by Paul and his companions. God did this so that they could be in His presence (glory) forever. That is how we all must come to Christ in that someone has to share the good news that Jesus died for our sins and rose again so that we may have eternal life with Him.
"So then, brothers, stand firm and hold to the teachings we passed on to you, whether by word of mouth or by letter." NIV translation
Paul reminds the believers of how they heard the gospel and how they were saved. Then, he urges them to hold onto what they were taught. This reminds me of the Parable of the Sower (see Matthew 13) and how, after some of the seed was sown, the birds came and ate it up. Paul is warning them to hold on to the truth and not let it be snatched away by men with all of their traditions and philosophies.
That is very good advice for us today as it seems like everyone has their own tradition and philosophy to try to replace the teaching of Jesus.
"May our Lord Jesus Christ himself and God our Father, who loved us and by his grace gave us eternal encouragement and good hope, encourage your hearts and strengthen you in every good deed and word." NIV translation
Paul prays that the Thessalonian believers would have the same encouragement and hope that he had because of Jesus and our eternal home with Him. We should all have that encouragement and hope as we know that the things here are only temporary. Many Christians walk around with very little hope and like they are mad at the entire world. That is not how we share the light of Jesus with the world. Those around us need to be able to see that we are different because of the hope that we have in all situations.