As Christians, we are all baptized into the Body of Christ. As Catholics, we all partake of supernatural food - the Eucharist - and drink a supernatural drink - the Blood of Christ - in the cup of communion. Yet neither our baptism nor our partaking guarantees us a spot in Heaven anymore than it guaranteed a spot for the Israelites in the promised land. There is quite possibly nothing less Biblical than the doctrine of once saved, always saved.
The Israelites were saved from slavery through the faith of Moses. This prefigured Christ's work of saving souls from slavery to sin. If the doctrine of once saved always saved held true, then all of the Israelites should have been able to enter the land promised to them through the covenant with Abraham. Yet not even Moses was permitted to enter into it because of his sin of failing to trust in God and to do as he was commanded to do.
Now, we as Christians are a very fortunate lot. We have the grace of confession, the ability to repent of our sins, confess them, and receive absolution. Unlike Moses, our sins do not have to be a permanent obstacle in the path of our salvation. We do not have to die in the wilderness unless we choose to. However, the key to receiving absolution of our sins is that we must not only repent of our sins but have the sincere intention of amending our lives. It is not enough to simply presume that we are saved and then continue on our path without concern for where that path is leading us.
God understands that we will fall many times on our journey home to Him. He understands that we are weak, that we are imperfect, and that we will falter and doubt and grumble and complain. This is why He gives us the graces we need to seek forgiveness when we become aware of our sin. Yet it is up to us to take advantage of that forgiveness, to seek out the sacraments and to avail ourselves of them so that some day, when our work here is done, we may enter in to that paradise at last.
It is the worst kind of presumption, though, to think that just by accepting the gift of salvation we are guaranteed a place in Heaven. This contradicts scripture. Christ himself said that not everyone who called upon his name would have a place in Heaven. In Matthew 7: 21 Christ explicitly says, "Not everyone who says to me, 'Lord, Lord,' will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven." Furthermore, in Matthew 25:31-42, He talks about the day in which he will separate those who are sheep from those who are goats based not upon the confession that Jesus Christ is Lord, but upon the works they have done for their fellow human being.
“When the Son of Man comes in his glory, and all the angels with him, he will sit on his glorious throne. All the nations will be gathered before him, and he will separate the people one from another as a shepherd separates the sheep from the goats. He will put the sheep on his right and the goats on his left.
“Then the King will say to those on his right, ‘Come, you who are blessed by my Father; take your inheritance, the kingdom prepared for you since the creation of the world. For I was hungry and you gave me something to eat, I was thirsty and you gave me something to drink, I was a stranger and you invited me in, I needed clothes and you clothed me, I was sick and you looked after me, I was in prison and you came to visit me.’
“Then the righteous will answer him, ‘Lord, when did we see you hungry and feed you, or thirsty and give you something to drink? When did we see you a stranger and invite you in, or needing clothes and clothe you? When did we see you sick or in prison and go to visit you?’
“The King will reply, ‘Truly I tell you, whatever you did for one of the least of these brothers and sisters of mine, you did for me.’
“Then he will say to those on his left, ‘Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels. For I was hungry and you gave me nothing to eat, I was thirsty and you gave me nothing to drink, I was a stranger and you did not invite me in, I needed clothes and you did not clothe me, I was sick and in prison and you did not look after me.’
“They also will answer, ‘Lord, when did we see you hungry or thirsty or a stranger or needing clothes or sick or in prison, and did not help you?’
“He will reply, ‘Truly I tell you, whatever you did not do for one of the least of these, you did not do for me.’
“Then they will go away to eternal punishment, but the righteous to eternal life.”
Whoever has ears, ought to hear.