Hi Tanveer,
Due to the sin committed by Adam and Eve, all men are born of a sinful nature - that is born in this body in a sinful nature. The following is an extract from the Bible where Lord Jesus is telling a man named Nicodemus how to enter the kingdom of heaven:
John 3: 3-6
3 Jesus answered him, “Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.†4 Nicodemus said to him, “How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?†5 Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. 6 That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.
*To be born of the water is to be baptised. To be born of the Spirit is to receive the gift of the Holy Spirit of God who will live in us and teach us to walk according to God's ways.
This means we must be born again, burying our old sinful nature and in this rebirth, we are made new and sinless.
Romans 6:4
4 We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.[color]
Secondly, you ask why the sacrifice Jesus made for us doesn't automatically grant us all access to heaven. Well, when Adam and Eve committed sin in the Garden of Eden, they received the knowledge of good and evil. God gave us free will to choose what we want to do. If he wishes, he could control us all and force us to do his will but he does not. If people choose to sin and commit evil, how can the most holy God accept them into his kingdom with the blemishes of this world? We must walk according to His ways. Our sins cannot be forgiven through our own good deeds - no matter how good we try to be, how can it ever match to the righteousness of God? This is why it is necessary to accept Jesus as our saviour.
I hope this has answered your question, dear brother. If you have any more questions, do not hesitate to ask and I shall rely on the grace of God to answer you.
God bless you.