By nature of human beings, we are united to Adam. But for the believer, we are now instead united to Jesus.
Being united to Adam we are born into sin and receive the condemnation for his sin resulting in death.
Being united to Jesus, we instead are justified and made right with God because of the righteousness credited to us. We now receive life now and eternally.
Where one act of Adam condemned the whole human race, so too one act of Jesus gives life to all who believe in him.