If a person cannot live carefree in sin and yet still sins, can this internal battle be looked to as a “proof” of sorts that he or she has the Holy Spirit within and is saved?
Yes, I think a believer’s feelings of remorse following the commission of a sin are a sign that he or she is saved, especially if the remorse leads the believer to confession (1 John 1:9). These feelings are caused by the conviction of the Holy Spirit.
In a moving personal disclosure Paul admitted to being unable to live a sin free life and ended by exclaiming, “What a wretched man I am! Who will rescue me from this body of death?” (Romans 7:18-24).
Even though he showed us that God does not hold our sins against us, but attributes them to the sin nature that temporarily inhabits our body (Romans 7:20), he was still filled with remorse at his inability to live a life without sin. It was only the realization that there is no longer any condemnation for those who are in Christ Jesus (Romans 8:1) that helped him tolerate his weakness.
That said, Paul never stopped admonishing us to live our lives in a manner pleasing to God. As long as we’re on Earth we’ll never be perfect, but being saved makes us members of God’s royal family (Galatians 4:4-7). We’re sons and daughters of the King of the Universe and we have a responsibility to act accordingly.
Paul never said we had to behave a certain way to receive or keep our salvation. What He said was, “Only let us live up to what we have already attained” (Phil. 3:16).