That’s called a cornerstone.

When building masonry buildings, builders set this stone first. Without it the whole building could be off track. The foundation would not be laid straight. Without a perfectly square cornerstone, the whole structure will be crooked and out of order. How is the cornerstone in your life? Is your foundation laid according to Christ? Is the structure of every aspect of your life built according to the perfection of Jesus? Check your cornerstone. If the building isn’t lining up then you may need to restart from the foundation up. Remember, things may become tough, but you can’t build a building from the top down. You have to first tear down the old structure and relay the foundation. Only if you start with the cornerstone of Christ will your life truly start to come together and line up the right way. Only then will you have a firm foundation that will last forever.

So then you are no longer foreigners and strangers, but fellow citizens with the saints, and members of God’s household, built on the foundation of the apostles and prophets, with Christ Jesus Himself as the cornerstone.
Ephesians 2:19-20 HCSB