Jeremiah 31:31-34 / Matthew 16:13-23
In the study of philosophy, there is this classical definition for man, and that is man is a rational animal.
That term is of interest in that as much as man has intellect and free will, he is also nonetheless, a part of the animal kingdom in that he also has needs and has instincts.
And being a rational animal, man also has the power of choice - the choice to believe in God or not, and also the choice to serve God or not.
The people of God in the 1st reading knew who God is and believed in God. Yet they also chose to disobey God and to be unfaithful to Him.
So even though God made a covenant with them, they broke it, and so God said in the 1st reading: They broke that covenant of mine, so I had to show them who was master.
Putting in bluntly, when man does not acknowledge God as his Master, then he becomes the master of his own destruction.
Yet God has written His covenant of love deep in our hearts, and hence we are also able to acknowledge Jesus as the Christ, the Son of the living God.
Yes, Jesus is our Lord and Master. Yet it is not just about words or ideas. We are to follow our Lord and Master in how He serves and suffers for the sins of mankind.
When we do not acknowledge Jesus as our Lord and Master and follow His Law of love, then we are not any more rational than just another animal.