Matthew 4: 17 From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand.

Christ had to suffer and to rise from the dead the third day, for that repentance and remission of sins should be preached in his name among all nations (Luke 24: 46-47). John 3: 16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.

John 20: 31 But these (the signs which Jesus did) are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.

1 John 2: 23 Whosoever denieth the Son, the same hath not the Father: but he that acknowledgeth the Son hath the Father also.
God the Father revealed himself in his Son Jesus Christ.

Proverbs 30: 4 Who hath ascended up into heaven, or descended? who hath gathered the wind in his fists? who hath bound the waters in a garment? who hath established all the ends of the earth? what is his name, and what is his son’s name, if thou canst tell?

Even if you start to keep all the commandments and live a perfect life, you are still a sinner, because you inherited the sin from Adam with your birth, the knowledge about good and evil. That is the reason why Christ told Nicodemus in John chapter 3, that you must be born again. The meaning of it is to have faith that Jesus is the Saviour of the world, that he saved you from all sins.