In reading the Puritans, it seems they believed that salvation was a free gift but then a Christian must work hard not to lose their salvation. Is this true?
Unfortunately, we are not experts in Puritan writing, but this is just as well, since Christians are not to search for the truth of our faith in the writings of other men, even other Christian men. Rather, we find such truth in the pages of scripture alone.
On the authority scripture, we can assure you that salvation is not obtained by human works nor do human works "keep" or ensure our salvation. Paul himself rebuked the Galatian church for thinking they could maintain their salvation or become "perfected" by their works:
Paul calls the Galatians "foolish" for believing that their flesh (i.e., their works) could contribute anything to a salvation that is by grace alone.
Those who believe a Christian can "lose" salvation do not understand the nature of salvation itself. When a man is saved by God's grace, he is given the gift of faith, according to Eph 2:8.
God delivers faith to the heart and prompts our confession of faith by means of His Spirit, Paul says:
Once the Spirit of God enters a man, He changes the man's nature forever, making him alive with the indwelling of God's spirit:
Paul says this new spirit within us is a permanent part of our being, as a pledge from God that He will complete the work He has begun:
Like a caterpillar becoming a butterfly, a person's nature is changed into something entirely different by the working of the Spirit, and that change can never be reversed. As Paul says:
Notice Paul says the Lord "controls" us and we are no longer recognized (or are known) as someone in the flesh. We are a new creature, like that butterfly that emerges from the cocoon, we can never return to our old nature.
Our salvation is a supernatural, spiritual change, not something obtained nor maintained by the thoughts in our head nor by the works of the flesh, but by the power of God.