It is true that Jesus gave his life on behalf of mankind. When Adam/Eve sinned, they lost perfect life for mankind, therefore all of their offspring were/are born imperfect . Because we are imperfect and fall short of God's glory, we need the sacrificial death of Jesus Christ to have a righteous standing before God. However, it isn't a given, we have to repent of our sins, the same way the first century Christians had to, and we have to live our lives in harmony with God's will and purpose to the best of our ability. We can't go on purposely sinning and have God's favor.
Romans 3:23 "For all have sinned and fall short of the glory of God,"
Acts 26:20 "but to those in Damascus first and then to those in Jerusalem, and over all the country of Ju·deʹa, and also to the nations, I was bringing the message that they should repent and turn to God by doing works that befit repentance."
I tell you that in the same way, there will be more joy in heaven over one sinner who repents than over 99 righteous ones who have no need of repentance.
saying: “Repent, for the Kingdom of the heavens has drawn near.”
Therefore, produce fruit that befits repentance.
I have come to call, not righteous people, but sinners to repentance.”
No, I tell you; but unless you repent, you will all likewise be destroyed.
No, I tell you; but unless you repent, you will all be destroyed, as they were.”
NWT 2013 jw.org