I have heard this expression most of my life growing up in the South. It refers to the unearned, unprecitented, meritorious gift of Jesus' ultimate sacrifice on the cross that opened the doors to heaven and how one's acceptance of that truth, combined with a contrition of the heart, will grant eternal life. It is a beautiful reality of all Christians that we should all give thanks for everyday. The next question is what happens next? Well, there is one tiny little declaration added to that reality that causes many a debate amongst theologians and neighbors alike. My discussion have all been in the latter category from the perspective of a common sense Catholic, thus I am on the side of Tradition that has always taught that our salvation is a gift that must be cherished, protected, and practiced else it will be lost. Why follow Christians reject this idea is baffling to me because it just makes common sense.
What other club, institution, or organization will allow its members to behave however they want once they get in? What other group allows you to stay a member when you become inactive? I was once a member of St Vincent de Paul Society within my parish and was very active in planning, participating, and promoting the group. After years of volunteering and my responsibilities increased elsewhere, my activity diminished within SVDP to the point that I was no longer considered a member who could be called upon. No one saw my past participation as justification for thinking of me as a current member. I went from being active, to associate, to contributing, to has anyone heard from him lately? If not, shorten the membership list. I may feel like I am a member and still have the Vincentian spirit, but I am still not a member. To belong, you have to regularly invest in it. Getting the invitation to join, having people welcome you at the door, and even having a benefactor pay your dues is a great start. Sitting on your duff, missing all the meetings, and doing nothing with what you have been given is not a proper response and is more than just frowned upon.
Getting back in can be as simple as answering the call. Refusing to answer the door or pick up the phone blocks the invitation, no matter how earnest the desire for your return.
How much more of a gift have we received when we get invited, welcomed, and dues paid that we were incapable of paying ourselves? We did not even ask for the dues to be paid, yet they were. All we had to do is show up. Show up and then listen to what the Club President would have us do as good participating members. It only makes sense. It is just common sense.
When Jesus condemns to hell those in Mt 25 who were looking for their reward, He is saying that not only will you be judged for what you did do, but also for that which you neglected to do. Even though they knew him and He knew them, Jesus says depart from me you wicked. He did not judge them as much as gave an accounting for their actions, or lack thereof. His Church is going out into the world to change it and you have to keep up or fall behind. Get too lazy and you are lost altogether. Our Lord will not be neglected, abused, and taken for granted by a false doctrine that claims immunity.
Calling upon Scripture quotes such as "For whosoever shall call upon the name of the Lord shall be saved" is a good defense of once saved, always saved. The common sense Catholic says, "Yes, and… there is more good news" not either or, faith or works; it is faith and works through Grace.
Why else would Paul say I have fought the good fight, have finished the race? What race? Weren't you saved way back when with your face in the dirt drenched in shame, humility, and repentance? Why all the anxiety and endurance of a race? Because, Paul knew his conversion was just the beginning of his spiritual walk (sometimes run) with the Lord. He had to make the journey to strengthen his relationship and love of Christ. His course was salvation and he was competing against the evils of the world to persevere in it. St. Paul was interested in little else. Comparing salvation to a race may not be the best analogy, but it is what he chose and I believe him over my neighbor.
There is so much to say on the subject and more proof-texts in scripture on this particular subject than just about anything else. I come across such references regularly and I hope to fill in more here over time. Until then, let Matthew 16 be the standard: "The Son of man will repay everyone according to his conduct"