I must do now what scholars rarely do, but should do: I must define my terms. That way, even if you disagree with what I think, at least you'll know what I mean.
"Knowledge" is justified belief. That is, knowledge is consent to a true idea because the reasons for it are sound. Consent to things true for the wrong reasons, or for no reasons whatever, is not knowledge but luck -- dumb luck.
"Truth" is the agreement of our ideas with reality. If our ideas comport with how things really are, they are true. If they do not, they are false.
"Objective" is when the object under review controls your thoughts about it. "Subjective" is when the subject doing the investigation controls his or her own thoughts about the object. "Objective" receives from the object; "subjective" imposes upon the object.
"Certitude" is how sure we feel about a thing.
"Certainty" is how sure a thing is in itself, regardless of how we feel about it.
Regarding the last two notions, I need to say more.
Having great amounts of certitude does not make a thing certain. Having no certitude about a thing does not make it uncertain. We often mistakenly think so because we confuse these two concepts, which I'd like to keep separate. Because we conflate or confuse these concepts, we sometimes do things that increase our certitude about an idea and think that by doing so we have increased its certainty when we have not. Certitude is something about us; certainty is something about it. That's another way of saying that it's possible to feel quite sure about things that are utterly false and unsure about things that are objectively true. Think here about all the centuries we thought the earth was both flat and the center of the universe. To our detriment, we sometimes confuse and conflate things that are different, things like certitude and certainty. Our task, in part, is to keep separate things separate. Certainty and certitude are two such things.
I think that when many persons complain about uncertainty they actually are complaining about imperfect certitude. Therefore, when they set about trying to fix what troubles them, they do things that make them feel more certitude, but that don't really strengthen certainty. Some appeals to authority and to infallibility strike me as just such aids-to-certitude masquerading as aids-to-certainty. Those appeals are futile simply because we human beings cannot produce certainty. Producing certainty is a Divine, not a human, prerogative. Only God can make things certain, and He does so by establishing those things Himself. What God has established is certain -- whether we know those things or not, whether we agree with those things or not, and whether we have certitude about them or not.
Because I am a Christian and move in predominantly Christian circles, when I explain here the difference between certitude and certainty, and the way some folks strengthen the former while thinking they are strengthening the latter, two groups come to mind: Protestants who misapply the doctrine of Biblical inerrancy, and Catholics who misapply the doctrine of infallibility. I am not here arguing against either doctrine. I am remarking about their misuse and about the motivations that seem sometimes to lurk behind that misuse as its operative and generative principle. (To be fair, I lay exactly the same charge, and perhaps more, against most of the scientists with whom I have ever spoken. Indeed, I know of no group of scholars more inclined to conflate certainty and certitude than modern scientists.)
To move one step forward: The only good reason for believing a thing is because it's true; the only good reason for rejecting something is because it's false. That much is simple, but not easy. It's sometimes hard to tell which one is which, and that can be enormously unsettling. It undermines our certitude, which makes us deeply uncomfortable. Because we humans have a strong distaste for ambiguity, especially on the issues that are most important to us, we don't want to stay in that uncomfortable and unsettling position. To escape it, we sometimes produce specious reasons for believing things, reasons that comfort us, but that aren't sound reasons for belief, and that don't establish certainty.
Short of Heaven, full certitude is probably beyond us, even if we pay close attention to Scripture, to tradition, and to the Church. But I, for one, will not complain to God about that. If that is the world He left us, it is enough.
Knowing is required of us by God Himself. We are commanded by Christ to love God with all our heart, soul, mind and strength (Matt. 22: 37). When Jesus said that, He was quoting an Old Testament passage (Deut. 6: 5) -- a passage to which He added a word -- and the word He added was "mind." Minds are for knowing, and knowing is more than repeating what we've been told or taught. As a matter of Christian stewardship, we are to make the best use we can of our minds. By means of them we are to know -- each of us. Our task is to know things as they really are, and not how we'd like them to be, because how we'd like them to be does not make them more certain but simply gives us a heart-warming sense of certitude. As regards you and me, this shortcoming is not the shortcoming primarily of others. We need to see it as our own and to guard against it diligently. You will know yourself in greatest danger of opting for certitude (how you feel) rather than for certainty (how God has actually worked) in those moments when your system is under attack.
To quote Tennyson:
"Our little systems have their day;
They have their day and cease to be:
They are but broken lights of thee,
And thou, O Lord, art more than they."