The Roman Catholic Church teaches that there is something called the Sin of Presumption, which states that it is a sin for one to believe that he or she is assured of his or her salvation.
It seems to follow, then, that one must be, at least to some degree, uncertain of his or her salvation.
What is the basis for this doctrine, including a biblical basis?
This question actually touches on two topics.
Firstly, and most directly, it's asking for some elaboration on the Church's teaching about the vice of presumption. From the Catholic Encyclopedia,
Presumption is here considered as a vice opposed to the theological virtue of hope. It may also be regarded as a product of pride. It may be defined as the condition of a soul which, because of a badly regulated reliance on God's mercy and power, hopes for salvation without doing anything to deserve it, or for pardon of his sins without repenting of them.
Reading onward, several examples or types of presumption are presented. Ultimately, the "sin of presumption" is the presumption that God will forgive all sins, regardless of the sinner's disposition towards those sins, or with minimal effort in repenting. Or, in some particularly twisted cases, one might presume all of their own sins will be forgiven, somewhat regardless of their effort and sincerity in repenting.
So, while I can't offer an official list of biblical references to support this teaching, I think, when clarified as such, it becomes more obvious where the biblical basises and echos of this teaching are. I think this is most obvious in cases where Christ forgives someone:
Then Jesus straightened up and said to her, “Woman, where are they? Has no one condemned you?” She replied, “No one, sir.” Then Jesus said, “Neither do I condemn you. Go, [and] from now on do not sin any more.” (John 8:10-11)
Christ forgives the woman's sins and then commands her to sin no more. Forgiveness comes with a demand for repentance. Presumably, we can reject that forgiveness by neglecting to repent.
From a more theological basis, the Kingdom of God entails operating within the confines of God's will. In fact, the delight of God's Kingdom is precisely in God's will. Therefore, if we're not actively seeking to refine ourselves into alignment with God, it's not God's Kingdom we're really after. It's our own personal kingdom -- hell, in a word. And if hell is what we're actively working towards, forgiveness is moot; we're not asking for forgiveness.
So, I think the Catholic Church is trying to make it clear, by this teaching, that lip-service and half-hearted efforts are an indication that the sinner isn't really interested in God or God's Kingdom. It's an indication that the sinner is self-interested and self-loving to the point of believing forgiveness is only a formality or a form they need to fill out so they can enter the gate, allowing the rest of God's Kingdom to bask in their own personal awesomeness. And in such a state, while God may still be perfectly willing to accept this charming soul into His kingdom, that charming soul has no sincere interest in being there.
Secondly, you've noted a sense of uncertainty about one's own salvation. And while I don't know that I could easily locate sources for this, it is a general attitude of the Church that each individuals salvation is uncertain. In general, we claim a great deal of ignorance as to who and how many are saved. Is Hitler in hell, for instance? We have no idea. Some individuals within the Church are pretty certain he is; others are hopeful he isn't -- that no one is.
This concept can then be extended into the sin of presumption. In addition to the requirement that we acknowledging that all salvation is an unwarranted gift, it's also a mystery. The Church may believe itself to hold the "keys" to the Kingdom of Heaven. But, by no means can it force anyone through the gate. And we, as individuals, must acknowledge regularly that we get lost, mis-informed, etc.
It's seems it's precisely at the moment we attain satisfaction with our triumphs against sin that we're in the greatest danger of damnation. So, we do well to proceed with some uncertainty.
In I Corinthians 9:27, St. Paul says that he chastises his body and brings it into subjection lest, having preached to others, he himself should be rejected. With St. Paul apparently not certain of his salvation, I'd need an awful lot of chutzpah to be certain of my salvation.
THE COUNCIL OF TRENT Session VI CHAPTER XII RASH PRESUMPTION OF PREDESTINATION IS TO BE AVOIDED
No one, moreover, so long as he lives this mortal life, ought in regard to the sacred mystery of divine predestination, so far presume as to state with absolute certainty that he is among the number of the predestined, as if it were true that the one justified either cannot sin any more, or, if he does sin, that he ought to promise himself an assured repentance.
For except by special revelation*, it cannot be known whom God has chosen to Himself.
*St. Francis of Assisi received this revelation
