5 What then is Apollos? What is Paul? Servants through whom you believed, as the Lord assigned to each. 6 I planted, Apollos watered, but God gave the growth. 7 So neither he who plants nor he who waters is anything, but only God who gives the growth. 8 He who plants and he who waters are one, and each will receive his wages according to his labor. 9 For we are God's fellow workers. You are God's field, God's building. 10 According to the grace of God given to me, like a skilled master builder I laid a foundation, and someone else is building upon it. Let each one take care how he builds upon it. 11 For no one can lay a foundation other than that which is laid, which is Jesus Christ. 12 Now if anyone builds on the foundation with gold, silver, precious stones, wood, hay, straw— 13 each one's work will become manifest, for the Day will disclose it, because it will be revealed by fire, and the fire will test what sort of work each one has done. 14 If the work that anyone has built on the foundation survives, he will receive a reward. 15 If anyone's work is burned up, he will suffer loss, though he himself will be saved, but only as through fire. (1 Corinthians 3:4-15 (ESV)
“For no one can lay a foundation other than that which is laid, which is Jesus Christ.”
Paul likes to switch metaphors and similes, jumping from one image to another. Here he goes over from an image of farming to that of building. In both images the people working are united. They have the same plan, the same goal. But different people do different things. In this case they have different jobs assigned to them. There is a division of labor in the church. But there is only one foundation, Jesus Christ.
Of course, the foundation is the most important part of any building. It determines many different aspects of the final project, the shape of the building and even the weight and height of the building, to some extent even the types of material to be used in the building process. These at least are taken into account when the foundation is being planned.
This is all to say, the foundation isn’t you. If the foundation is you, then you become the pastor’s project and law becomes the tool as it is for any number of self-help gurus. But if Christ is the foundation then it is the gospel, the forgiveness of sins that is the tool of choice as it was for Paul who knew nothing but Christ and him crucified.