The words for sealing in the New Testament have multiple meanings. First of all, you can seal a document to protect it from tampering. You can do the same with a tomb or a prison cell. When you seal a document or a place, you are concealing something or someone: examples in the Bible include the tomb of Jesus (Matt. 27:66); the heavenly scroll (Rev. 5); and Satan's confinement in the abyss (Rev. 20:3).
Second, sealing can certify that something or someone is reliable: certified letters have a seal indicating that the information inside is trustworthy or has been delivered without tampering (cf. John 3:33; 6:27; Rom. 15:28; 1 Cor. 9:2).
Third, sealing can indicate that God has accepted someone. The Lord knows who belongs to Him, and He gives them the Holy Spirit (2 Tim. 2:19; 2 Cor. 1:22; Eph. 1:13; 4:30). Sealing, therefore, became associated with circumcision in the first century (Rom. 4:11) and baptism in the second.
The sealing of Revelation 7 occurs just before the close of probation. From John's point of view, sealing involves how people relate to God at the end. In the broader sense, sealing is the indication that people are acceptable to God. "The Lord knows those who are his" (2 Tim. 2:19, NKJV). So the New Testament does not limit the concept of sealing to the end-time. But in Revelation 7 sealing occurs in an end-time setting. The final proclamation of the gospel results in a great last-day sealing work.
This kind of study doesn't make for light reading. Sometimes Christians need to do heavy, detailed investigations of Scripture in order to understand God's ways. We live in an age that prizes relevance more than learning, and many people have no patience with Bible study that doesn't have an obvious and immediate payoff.
But I suggest that such investigation can lead to great joy. If Christians want to understand the deep things of God, they must at times do detailed study, even though its immediate usefulness may not be clear. But the long-term reward for such study is an understanding of the big picture that transforms everything you read in the Bible.
Lord, thank You for the depth of the challenge in Your Word. Encourage me to take up that challenge and open my mind to comprehend the immensity of Your wisdom.