^3:3 So remember what you received and heard. Obey, and change the way you think and act. If you’re not alert, I’ll come like a thief. You don’t know when I will come.

So remember what you received and heard.

From whom did they receive and what did they hear? They received instruction from the apostles and other prophets; that is what they heard...Bible was not written down as such for the church members to each have a copy. Thus, it was by hearing the truth (not the misleading religion like Jezebel was perpetrating at Thyatira).

Obey, and change the way you think and act.

What were the people of Sardis church to obey?

Most likely they should have obeyed the Word of God. But, how many churches currently have attendees who obey the Word of God (Rev. 21:1-8)?

The action of "change the way you THINK and ACT", is different for each person. We, each, have those "protected areas" in our lives, and we don't want to be obedient to remove them. It, thus, takes a personal commitment to "change the way you think and act."

If you’re not alert, I’ll come like a thief. You don’t know when I will come.

While this may refer to Christ's return (the day of the Lord as recorded in 1 Thessalonians 5:1-6), more likely this refers to the unpredictable time of DEATH. Remember this church was in view of the necropolis (burial mounds) and apparently was preoccupied with death rituals.

PHYSICAL DEATH stops any chance to "OBEY, and change the way you think and ACT.

IT IS TOO LATE WHEN YOU DIE!!!!!