The term translated as “what” actually means “how.” The Lord does not call on the Christians of Sardis to remember what they received and heard (that would be a reference to the content of the gospel), but calls on them to bring to mind how they received and heard the gospel (cf. 1 Thessalonians 1:6; Acts 17:11). As they now are “dead” and need to “wake up” (i.e., be watchful), they need to think back to their initial enthusiasm for the good news of Christ’s work and replicate that zeal.
3 Remember, then, what you received and heard. Keep it, and repent. If you will not wake up, I will come like a thief, and you will not know at what hour I will come against you.