Gnostics claimed there was no such thing as sin, therefore it was impossible to sin, for it did not exist. There was no point in seeking not to sin, because there was no such thing as “sin” or “sinning” in the first place. John strongly contested such thinking.
Necessarily, thieves do not announce their expected time of arrival, as that would be counterproductive. In the same way, Jesus’s return is certain and imminent, but kept under wraps.