“[…] Just as you are in fact walking, we request you and appeal to you by the Lord Jesus to keep doing it more fully.”
1 Thessalonians 4:1

The members of the Christian congregation in Thessalonica were not perfect.
They had moral standards and love, but could improve on both counts. (1 Thess. 4:3,4,9,10)
That is why Paul commended them while tactfully encouraging them to “pursue what is good toward one another.” (1 Thess. 5:15)
Regardless of how long it has been since we became Christians, ‘making sure of all things’ and ‘holding fast to what is fine’ is something we have to remember to do every day. (1 Thess. 5:4,6,8,21)
We cannot afford to take our faith for granted, and as long as we are imperfect, there will be things we can improve on.