Answers from our SPSE Catholic Apologetics Facebook page:

Catholics can have a level of certitude, but it isn’t absolute. No Protestant I know actually believes in complete assurance either. That fellow they THOUGHT was saved but then sinned … well, he wasn’t *really* saved to begin with. Are you sure you are now sinless and ready for heaven?
Michael C

We’ll you would have to explain why from chapter 1 and up to chapter 5 there are 25 ‘if’s’:
1:8 If we say…
2:1 if we sin…
2:3 If we keep his commandments
2:4 If someone says…
2:5 if you keep his word
2:6 If someone says…
2:15 If someone loves the world…
2:19 if they had been one of ours…
3:20 if our conscience reproaches us…
That’s a lot of ‘If’s?
If you examine this chapter carefully you cannot come to the easy conclusion that you imply in regards to 5:13…
Jose R

The Greek word for knowledge in 1 John 5:13 does not necessarily equate to absolute certainty. We use the verb know the same way in English. For example, I may say I know I am going to get an A on my exam tomorrow. Does that mean I have metaphysical certainty of this? Not at all. What I mean and what the verb know can be used to indicate is that I have confidence that I will get an A on my test tomorrow because I have studied the material thoroughly and I know it well.
Gary G