Jesus made it impossible to be a Child of God in the traditional Jewish way. Jesus preached the Laws and the requirement to follow the spirit of the Laws as well as the letter of the Laws. The previous posts about the Sermon on the Mount outlines these requirements. It was Paul who understood the real mission of Jesus. It was not to preach stricter adherence to the laws but to fulfill the stricter requirements of the Jewish Laws that called for the payment or punishment of sins. Jewish law required a blood sacrifice for the redemption of their sins. And Jesus came to pay that price!
It became Paul's task to explain that sacrifice to Jews and Gentiles alike. While studying with the disciples and others who knew Jesus, he began to develop the theology we now recognize as Christianity. The Gospel of John has the most concise explanation of this new faith. "...that whosoever believed in Him, shall be saved..."
(John 3:16). Paul developed it further in his letters, especially in his epistle to the Romans.
We humans simply can not do what the Laws required. We lust; we hate; we steal; we have other things as our gods; we are envious; in fact, we are incapable of "...being perfect..." We are all doomed to the fires of Hell as punishment for breaking the Laws of God. Those Laws require a fitting punishment. Think of the
criminal who is convicted of breaking our laws and is sentenced to a number of years of confinement or even death. The criminal has been tried and convicted and must serve the punishment. This was the system developed by the Hebrew people. Paul understood this system. Until he had his conversion experience some 5 years after Jesus was crucified, Paul persecuted those who would dare to suggest this system was not correct. After his conversion, he had a new passion, spread the good news of the freedom offered by the sacrifice of God's only son, Jesus.
In the Book of Romans, Paul explains this concept fully. I can not do justice to Paul's arguments. I suggest you read the sources. 1. The Bible 2. Book of Romans in Wikipedia
So personally, I believe I am saved from the punishment that I deserve because I believe God sent Jesus to receive the punishment for my sins. Because of this faith in the substitution of Jesus for me, I am assured of my place in the family of God. My task now is to learn what it means to be a Child of God. How should I act; how can I represent God in such a way that others will also want to be a Child of God also.