Paul is saying is once you have received Christ you are of the day and you are of the light. Listen to this, I am making the statement, I will own up to it. It is impossible for a Christian - a born-again believer, one that has received the Spirit of Christ - to be left behind in the day of the Lord's coming. I have heard the illustration before, "If you are a believer and you are not ready you could get left behind." No, it is impossible. What makes you ready is the fact that Christ is living inside of you. Not that you believed in Him with your mind. But that you received His Spirit into your heart, into your spirit. That is what makes you ready when Christ comes so that day will not take you unawares.
These commands that we have studied are all present imperatives. Present tense commands, which means these are things that we are to be continually and presently doing. This is what we are to be doing while we are waiting for the coming of the Lord. This is what it means to be watchful and thinking soberly and not allowing the world's philosophy and its activities to pull us away as we wait for the Lord's return.
The word sanctify means to set apart. When the Holy Spirit comes into a person's spirit that person is set apart, sanctified, belonging to Christ. But as Paul says, May the God of peace Himself do it, may He sanctify you completely to the end. Now that phrase completely to the end is one compound Greek word. The first word is the word whole, and the second word is the word end. So, May He keep you whole all the way to the end. He is not saying, "May He sanctify you and then gradually progressively sanctify you all the way to the end." Rather the text reads, "May He make you complete and then keep you all he way to the end this way." Completely to the end.