Jesus is our King (we Catholics). We are convinced simply by his saying so.
But, there was another man who needed to be convinced before he would take Jesus as his word. His name was Nicodemus. This man came to Jesus one night and said to him, “Teacher, we know that you are a teacher who is come from God, for no one can do these signs that you do unless God is with him.”
Jesus answered him, “I am telling you the truth, unless one is born again from above he cannot recognize the kingdom of God.”
Nicodemus smiled, thought for a moment, then asked, “How can a man be born when he is old? Can he enter a second time into his mother’s womb and be born?”
Jesus answered this problematic doubt: “Again, I am telling you the truth, unless one is born of water of physical childbirth and then born of the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.".
Basically, Bahman, Jesus (in the acts of his People), gives this new birth into the Kingdom of God to anyone who seeks it. His Spirit is given to them immediately and instantaneously when they are baptized by his People, so that they are “born of the Spirit”. They are enlightened, a light shining within their souls, to now see and understand in their intellect and will what they could not see before. On a full moonlit night, one can see objects, shapes, clearly enough to walk. But when the sun rises, colors become visible because of the addition of direct light. It is the same with us who have the Holy Spirit; we understand things are true that were not visible with only our natural reason and intellect. Many people seek enlightenment, because they want to understand; but in Jesus enlightenment is brought immediately to us. Many people do not want this light because they already, at a level of human reason, want to remain in moonlight, where truth is not known, able to continue believing things they want to believe.
[John chapter 3 is location of the encounter of Nicodemus with Jesus]