According to the Bible, the slaves of God are sealed when they receive the Holy Spirit:
In Him you also, when you heard the word of truth, the gospel of your salvation—in Him when you believed—were sealed with the promised Holy Spirit.
Eph 1:13 (HCSB)
Revelation 7:3 is often used by the WTBTS to support their position that Ephesians 1:13 refers only to an 'initial sealing' and that Revelation 7:3 refers to the 'final sealing':
“Don’t harm the earth or the sea or the trees until we seal the slaves of our God on their foreheads.”
Rev 7:3 (HCSB)
w97 8/15 14-15 Living for Today or for an Eternal Future?
12 Consider the text at Revelation 7:1, 3, which says that the “four winds” of destruction are held back “until after we have sealed the slaves of our God in their foreheads.” This is not referring to the initial sealing, which takes place when those of the 144,000 receive the heavenly calling. (Ephesians 1:13) It refers to the final sealing, when they are irrevocably identified as tried and faithful “slaves of our God.”
What scriptures do they use and what is their reasons for supporting this doctrine of an initial sealing and a final sealing, when the cited scriptures do not refer to an initial or a final sealing, other than to reinforce the opinions of celebrated WT scholar J.F. Rutherford that there are two different classes of citizen Christians in God's Kingdom?