Washing robes white in the blood of the Lamb is observing Christianity which was achieved by Jesus blood.
13 And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they?
14 And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb.
15 Therefore are they before the throne of God, and serve him day and night in his temple: and he that sitteth on the throne shall dwell among them.
16 They shall hunger no more, neither thirst any more; neither shall the sun light on them, nor any heat.
17 For the Lamb which is in the midst of the throne shall feed them, and shall lead them unto living fountains of waters: and God shall wipe away all tears from their eyes.