The Second Coming of Christ was a major emphasis of Paulâ€™s teaching to the Thessalonians (in fact, Paul references it no less than 8 times in this epistle) as well as a major theme in Scripture. As Christians, we are to have an eschatological bent to our outlook such that in all things we are anticipating the return of our Lord- that is THE Christian hope (Rom. 5:2; Col. 1:27; Tit. 2:13)!
Combining the language here, at the end of this age there is going to be a Cosmic Worship Service in which we, Godâ€™s people, will be presented to God as a holy sacrifice acceptable to the Lord. As such, Paulâ€™s passion for the Thessalonians was that in and through all things they guard their hearts such that they remain an â€śundefiled offeringâ€ť of Christâ€™s to His Father!