The Gospel message (including the books of Acts, Hebrews and the first several chapters of Romans) is for the entire world (Mark 16:15)
The NT Epistles are written instruction to members of the early Church. From them, we may continue to glean godly insight and wise instruction as we await Christ’s return.
~Everyone, regardless of how long they have claimed the label “Christian”, can benefit from hearing the true Word of salvation. (Revelation 14:6-7)
~It is acceptable to help others see their need through “lawful” use of the OT Law (Romans 7, 1 Timothy 1:8) …maybe will come back to this thought, later
~It is acceptable to (respectfully) share with an unbeliever the reason for your faith (1 Peter 3:15-16). Personal experience can be very convincing evidence. The OT narrative offers a record of our tendency to be wrong. Paul provided an explanation of our desperate spiritual need in the early chapters of his letter to the Romans.
~It is acceptable to work out God’s love at a social level to help counteract the injustices of society (Galatians 6:10; Philippians 4:5; Romans 13:7-9)
~It is not remotely helpful for Christians to attempt to direct the behavior of the unconverted through external moralization which is based on Christian principles. In fact, Jesus soundly condemned this practice when He spoke “woes” over those who traveled land and sea to make a single convert…only to sink that person even deeper into a state of spiritual lostness (Matthew 23:15).