The Apocalypse of St John chapter 7 tells us:

These are they who are come out of great tribulation, and have washed their robes, and have made them white in the blood of the Lamb. [15] Therefore they are before the throne of God, and they serve him day and night in his temple: and he, that sitteth on the throne, shall dwell over them. 

And chapter 15 tells us
[1] And I beheld, and lo a lamb stood upon mount Sion, and with him an hundred forty-four thousand, having his name, and the name of his Father, written on their foreheads. [2] And I heard a voice from heaven, as the noise of many waters, and as the voice of great thunder; and the voice which I heard, was as the voice of harpers, harping on their harps. [3] And they sung as it were a new canticle, before the throne, and before the four living creatures, and the ancients; and no man could say the canticle, but those hundred forty-four thousand, who were purchased from the earth. [4] These are they who were not defiled with women: for they are virgins. These follow the Lamb whithersoever he goeth. These were purchased from among men, the firstfruits to God and to the Lamb: [5] And in their mouth there was found no lie; for they are without spot before the throne of God.

These are those who have devoted themselves totally to The 
Lamb, Jesus Christ and his Father and His Name is written on their foreheads.

The number 144000 is symbolic of 12 (tribes of Israel) X 12 (Apostles) X 1000 (a multitude). 

Then is it too late for someone to return to the church if people have been marked?

What do mean by "people have been marked'?
Marked by what?
In reality "too late" means "already dead". A dead person cannot return to the Church.
We have one life to live.
We have our whole life in order to prepare to meet our God.
Maybe you can fill out your comments with a concrete situation or examples to explain what you mean.