John 3:3 "Jesus answered and said to him, "Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God."
and 1 John 1:9 "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness."
2 Cor 5:21 "For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him."
We can debate transubstatiation at another time
The first one is the why question of being born again. The how is answered here:
Mark 10:17-31 17 Now as He was going out on the road, one came running, knelt before Him, and asked Him, "Good Teacher, what shall I do that I may inherit eternal life?" 18 So Jesus said to him, "Why do you call Me good? No one is good but One, that is, God. 19 You know the commandments: 'Do not commit adultery,' 'Do not murder,' 'Do not steal,' 'Do not bear false witness,' 'Do not defraud,' 'Honor your father and your mother.' " F46 20 And he answered and said to Him, "Teacher, all these things I have kept from my youth." 21 Then Jesus, looking at him, loved him, and said to him, "One thing you lack: Go your way, sell whatever you have and give to the poor, and you will have treasure in heaven; and come, take up the cross, and follow Me."
Being born again is the first step of repentance -- shunning evils as sins. Which evils? Those set forth in the Ten Commandments. As I stated, all religions follow the Ten Commandments and indeed the commandments extend outside religion to civil law. Christian Conservatives fight to keep the Ten Commandments in the town square today.
As I said, being born again is a process. It does have a first iteration when a person decides to freely shun evil when he otherwise would not and then proceeds into the repentance process I described earlier. Note that this is not unique to Christianity.
If you have lived your life well, you have gone through the process of regeneration.
The next verse adds more context to the 2nd quote:
10 If we say that we have not sinned, we make Him a liar, and His word is not in us.
We are to confess our sins individually as the Catholics do, not wholesale like the reformed. I won't get into the whole forgiveness thing because we don't believe in 'original sin'. We believe in hereditary sin which is passed from parent to child, grandparent to parent, throughout the ancestors which are treated differently than sins of our own making.
Therefore, we do not believe in vicarious atonement.