"Let the little children come to me, and do not hinder them, for the Kingdom of God belongs to such as these. I tell you the truth, anyone who will not receive the kingdom of God like a little child will never enter it." Luke chapter 18 verses 16-17
Also in Luke Jesus states, "How hard it is for the rich to enter the Kingdom of God! Indeed, it is easier for a camel to go through the eye of a needle than for a rich man to enter the Kingom of God." (chapter 18 verses 24-25)
Luke relates that at the Last Supper, Jesus said, "I tell you I shall not drink of the fruit of the vine until the Kingdom of God comes." Luke chapter 22 verse 18
In the gospel of John Jesus states a fundamental tenet of Christianity concerning the Kingdom of God. "I tell you the truth, no-one can see the Kingdom of God unless he is born again." John 3 verse 3