Like the hidden treasure (Matthew 13:44), the pearl is an object of value. It can be part of a treasure, but there is a difference between "treasure" and "the pearl." Jesus says in verse 44: "like treasure." However, notice what He says in verse 46: "one pearl of great price." The difference is that "treasure" is a collective noun. Treasure is made up of many pieces of gold, silver, coins, articles of fine clothing, art, or gem stones. We can think of it like the treasure of a pirate in a chest buried somewhere in the Caribbean. That is what Jesus intends in the Parable of the Hidden Treasure—many valuable things in a collection. In the Parable of the Pearl of Great Price He considers one particular object of great value, the centerpiece of His treasure.
Emphasizing the oneness under God, Paul writes in Ephesians 4:4-6:
There is one body and one Spirit, just as you were called in one hope of your calling; one Lord, one faith, one baptism; one God and Father of all.
Note in particular "one body." Here Paul stresses the church's singularity, uniqueness, oneness. Christ has only one church. Paul mentions this in Romans 12:5: "So we, being many, are one body in Christ, and individually members of one another." We have unity even though the church is made up of many members. Not only that, its members are interdependent of one another. They rely on one another to do certain things within the body to make the body function as it is designed.

Paul continues the thought in Colossians 3:15: "And let the peace of God rule in your hearts, to which also you were called in one body; and be thankful." This verse focuses us on being called into one body, and God put us in it to do or be something specifically. In I Corinthians 12:12-14, 27, this is repeated. We are many members but one body, and God put us each in the body to do what He wants us to do.

The "pearl" is the church as a whole, whereas the "treasure" in the preceding parable is the church in its individual members. In the first parable, Christ is assuring us that He has His eye on us for ourselves—that we are immeasurably valuable to Him as individuals. However, in this parable, He switches the focus slightly to assure us that all of us as a body, His Bride, are important. We are the centerpiece of His treasure—the Bride who will marry the Son.
Ephesians 5:25-27 brings out the "bride" aspect:
Husbands, love your wives, just as Christ also loved the church and gave Himself for it, that he might sanctify and cleanse it with the washing of water by the word, that he might present it to Himself a glorious church, not having spot or wrinkle or any such thing, but that it should be holy and without blemish.
This gives an idea of how much value Christ places on the church. It is pretty high praise, a lofty goal, to be considered this way by Him. Once He calls us, He sets out to perfect us, to make us absolutely holy and without blemish, so we can be a fitting spouse for Him.
Let us be glad and rejoice and give Him glory, for the marriage of the Lamb has come, and His wife has made herself ready. And to her it was granted to be arrayed in fine linen, clean and bright, for the fine linen is the righteous acts of the saints. Then he said to me, "Write: Blessed are those who are called to the marriage supper of the Lamb!" And he said to me, "These are the true sayings of God." (Revelation 19:7-9)
What is the lesson? Christ joyfully gave His all for the church as His Bride, and He will prepare it as His adornment, just as a king adorns his clothing and crown with pearls. This should give us encouragement in our battle against Satan. We have so much going for us, not only as individual sons and daughters of God, but because we have been called right now as part of His Bride. If we keep up the good work, if we allow God to work in us and remove all our blemishes, what a glorious future we have!