Saint Paul Was Afraid Of Losing His Religion?

Today the first reading in Mass was from Saint Paul's letter to the Corinthians.  As you know the epistles were written as correction, exhortation and instruction. They were never written with the intention of being all there was to know of the new Christian faith. (That would come 1500 years later in the Catechism of Trent) As a matter of fact Saint Paul wrote several times to take heed to what was passed down orally, not just in written instruction.  So when he wrote the following lines of his epistle, what was going on in Corinth? What mis-information was he trying to correct or what behavior was he rebuking them for? We know the Corinthian Church had fallen into some pretty dark times with a whole lot of immorality going on, and terrible abuses of the Eucharistic meal as well. In the context of these abuses this is how he addresses the Church:

 I am reminding you, brothers and sisters, of the Gospel I preached to you, which you indeed received and in which you also stand. Through it you are also being saved, if you hold fast to the word I preached to you, unless you believed in vain.

Does Saint Paul reassure the Corinthians that no matter what, since they have indeed received and stand in the gospel, they need to do nothing further to ensure their salvation?

Does this verse indicate that saint Paul believed that salvation is a once and done event?

Is Saint Paul indicating that if you don't hold fast to the word, your initial belief in the gospel is in vain?

As Catholics, we don't live in fear of "losing our religion" but we do need to be reminded (unless we are somehow different than the Corinthians) that we need to hold fast to the word preached to us. That entails living a life in obedience to Christ trusting in His grace to empower us to do just that. As Saint Paul says, we are in the process of being saved, on the condition that we hold fast.