Jesus comes like a thief to the unsuspecting, the sleepy (Revelation 3:3). 

That doesn’t simply mean that His coming is a surprise. It means that He comes to take. That’s what thieves do.

To prevent this, the angel of the church at Sardis has to “keep” (tereo) or guard what he had received. 

Israel had to “keep” the statutes of Yahweh, and to guard the treasures of God’s house. They had to guard those against attacks of all sorts, not least the threat that Yahweh Himself might come and take His gifts back. 

A sleepy Israel would soon be a poorer Israel, as Yahweh comes to take from those who have but who fail to guard. In the end, that’s just what Yahweh did - He sent Nebuchadnezzar and later Titus to take away the treasures of Israel that Israel had become weary of guarding.