The Gospel gives us a synthesis of the history of salvation which describes the many times God tries to connect with his people through messengers, and finally sends his son Jesus who was also rejected. We all know this, but we also know how that applied to us when we reject God in our own way, preferring other things to God.
One of the many lessons in the Gospel is the one expressed in this verse: “The stone that the builders rejected has become the cornerstone”.(Ps 118:22)
The Son who was rejected became for the whole world the source of eternal salvation. In the same way, without knowing, some times we reject people who we later know are fundamental in our lives. Sometimes, directly or indirectly, we reject things that should lead us to God. For that reason, the gospel insists that we value Jesus as the cornerstone of our lives.
In ancient building practices, the cornerstone was the principal stone placed at the corner of the edifice. The cornerstone was usually one of the largest, the most solid, and the most carefully constructed of any in the edifice. Jesus describes Himself as the Cornerstone that His church would be built upon, a unified body of believers, both Jew and Gentile. 
Without the cornerstone, the building will fail, or will not be completed. In the physical building this is easy to understand. The same would happen without Jesus. Our lives would be incomplete and would not have a solid foundation. We the people of God are fellow citizens with God’s people and members of God’s household, built on the foundation of the apostles and prophets, with Christ Jesus himself as the chief cornerstone. ( Eph 2:19-20)
I invite you to reflect on how much we consider Jesus as the cornerstone in our lives, how important Jesus is in our lives and how much He is present. I also invite you to reflect in the life of the Saints and see how in them their lives are so powerful because of the conviction that Jesus is the cornerstone of their lives.