Can you lose your salvation

Rec 3:5 He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels."

The people in Sardis were in danger of losing their salvation through continued disobedience to Jesus (v3). Their name would be blotted out of The Lamb's Book of Life if they did not obey.

They had salvation otherwise they would not have been in Lamb's Book of Life.