Throughout the scriptures, we see apologetics in one form or another.
The explicit call to the practice is first found in the words of Peter. Revering Jesus as Lord, we prepare the argument but, as He lived with his humble and gentleness, we do this the same.
“But in your hearts revere Christ as Lord. Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect,” 1 Peter 3:15 NIV
Apologetics at the time of the original church heavily relied on arguing for the hope in the Gospel via first testament scripture that pointed to truths Saints lived by. Paul often did this via epistles.
Two examples of this are firstly found in the massive text of Acts 7:2-50 where Stephen essentially teaches a refresher course on the scriptures. But another, more brief example is found below:
“For I am not ashamed of the gospel, because it is the power of God that brings salvation to everyone who believes: first to the Jew, then to the Gentile. For in the gospel the righteousness of God is revealed- a righteousness that is by faith from first to last, just as it is written: “The righteous will live by faith.” Romans 1:16-17 NIV
Footnotes are a blessing for us who aren’t as familiar with the scriptures. We can clearly see this Old Testament base for Paul’s argument.
An exception to scripture basis is when the audience lacked prior knowledge of said scripture.
“The wrath of God is being revealed from heaven against all the godlessness and wickedness of people, who suppress the truth by their wickedness since what may be known about God is plain to them, because God has made it plain to them. For since the creation of the world God’s invisible qualities-his eternal power and divine nature-have been clearly seen, being understood from what has been made, so that people are without excuse.” Romans 1:18-20 NIV
In other places Paul even quotes non-believer writers popular to the culture of his audience, using their work as a vehicle to get them to the destination of Christ.
For us who seek to follow the call Peter gave to give the answer the hope we have, we have the position of nature, and even most of the social sciences that are just now catching up, coming to their senses seeing the truths many of the things the Word has said for so very long.
But that’s for mom believers. Now, as sad as the very fact that people can lack faith in God is true, that’s not the worse thing. The worse thing is when the believer in God need to be taught or have simple truths in the very word they claim to live by used to argue for the way they should live. This was the case with the Pharisees, and today with many lost sheep.
This is essentially preaching. Christians today use both Testaments to often argue why we have this hope and live by it, and why other Christians should, too.