The word, earnest, only occurs three times in the Scriptures.
The Greek word translated earnest signifies a pledge, money given in token of a bargain made. For illustration, when a house is purchased, it is customary to pay down a certain percentage of the purchase price to show that the purchaser means to complete the bargain. When the seller gets the percentage, or earnest, he knows that he will see the lump sum later on.
In a similar way we, Christians, are in this world of sin and suffering and discipline. We are strangers and pilgrims on our way to the heavenly home where all shall be sinless and stainless. We have not arrived yet, but the pledge that we shall be is given by the Holy Spirit. In other words by the Spirit's power we taste now the joys that shall be ours in all their fullness when we get to glory.
We remember hearing a good illustration of the seal and earnest of the Spirit. Suppose a farmer goes to market and takes one of his men with him. He buys a few sheep, and gives orders to his man to put his mark upon them, so that they can be identified, if they get straying, or mixed up with sheep belonging to other owners. That is like the sealing. The sheep are bought and the farmer instructs his man to drive them home. On arriving it would be dark, so he instructs that they should be placed in the empty barn for the night, and each given a few handfuls of clover to eat, and on the morrow put in the clover field. Suppose the sheep could understand and express their thoughts one to another, would they not say as they were enjoying the clover in the dark barn, " If the clover tastes so good in the dark barn, how good it will be to be in the clover field in the bright sunshine tomorrow?" So, if we Christians enjoy the heavenly food in the dark barn of this world, what will it be when we are in our proper sphere in the Father's house on high, enjoying these things without let or hindrance? That is like the earnest. The following incident well illustrates this.
A good many years ago a Christian conference was being held in the United States of America. A lonely isolated Christian, who had a very few opportunities of Christian fellowship, traveled a good thousand miles to be present. When he saw more Christians gathered together than he had ever seen before probably, his spirit was moved, and he asked the assembled Christians to sing the hymn beginning with the words,
" What will it be to dwell above,
And with the Lord of glory reign,
Since the blest knowledge of His love,
So brightens all this dreary plain?
No heart can think, no tongue can tell,
What joy 'twill be with Christ to dwell."
The working of the earnest of the Spirit was plainly seen in what was filling his mind with joyful anticipation.