The Catechism in Questions & Answers

565. What is the significance of the images in Revelation 14: 1-5?

The mark (“seal”) with the name of the “Lamb” and of the Father indicates that the hundred and forty-four thousand are the property of God.
Being “without fault”, having “no deceit” in their mouths, and “following the Lamb”, indicate that they have led their lives in accordance with the gospel in word and deed.
The term “firstfruits” describes those whom Christ will take to Himself at His return: these will be the first to receive complete salvation.
The “Lamb” refers to Jesus Christ: “Behold! The Lamb of God who takes away the sin of the world!” (John 1: 29). Revelation 5: 12 relates that the Lamb who was slain has gained the victory. This means that the abased and crucified Son of God also triumphs and conquers.

Salvation: see Questions 243. et seq.

“Worthy is the Lamb who was slain to receive power and riches and wisdom and strength and honour and glory and blessing!”
Revelation 5: 12