[Sin and Grace] ... a person ... who says they believe ... lead a lifestyle that is totally opposite ...
Q:What do you say to a person, family member, co-worker, etc. who says they believe in God ("so I will go to heaven") but lead a lifestyle that is totally opposite and doesn't reflect any walk with Christ at all?
A:It is great to hear that you are open to speak about your faith. Regarding someone of faith leading, "a lifestyle that is totally opposite and doesn't reflect any walk with Christ at all", you are right, God's Word is clear when it comes to our everyday actions as Christ followers.
The Bible says: "Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind." Romans 12:2a (NIV) (*See also: James 2:17-22; Matthew 7:17-20, and Ephesians 4:17-24)
As followers of Jesus Christ, the Bible is clear - we are to adopt Jesus' way of life as our own. Jesus says in Luke 8:21, "My mother and my brothers are those who hear God's Word and put it into practice." When it comes to a life of faith, our deeds and our creeds (statements of belief) are both important.
This being true, something also important to remember is that following Christ is a journey, not a destination. We "grow as we go", not getting the life of discipleship right over night, but over time, as God's Spirit continues to refine us throughout life.
When looking at another's life, we also need to remember our own journey (Matthew 7:1-6). On account of our sinful nature, each of us has moments where our lifestyle "is totally opposite and doesn't reflect any walk with Christ." The Apostle Paul himself also knew this struggle (Romans 7:14-25). This is the primary reason we trust Jesus Christ for our salvation – everyone continually comes up short of God's standard of perfection.
As we grow in reflecting the reality of being God's children, I like the words of Ephesians 4:2b (NLT), "Always be humble and gentle. Be patient with each other, making allowance for each other's faults because of your love."