Because of their preaching verse 9 says there is a great multitude before the Lamb (Christ) having white robes and palms in their hands. This is a true picture of salvation and the number seems to be uncountable. Their cry is, "Salvation to our God which sits on the throne and to the Lamb." You will notice Christ (the Lamb) is recognized for who He is during that time. One of the elders asks where all these come from and John answers, "You know." Then the elder answers, "These are those who came out of great tribulation and made their robes white in the blood of the Lamb. (Christ) So the blood of Christ is at work here too, as it is in every age.