While truth is absolute, any one human being’s knowledge of the truth is relative in certain respects.
First, knowledge of the truth is not exhaustive knowledge of all truth. Human knowledge is relative in content. Our opportunities, our intellectual ability, our interests, our teachers, and our presuppositions all influence which particular truths we come to know. Which particular facets of a jewel we see depends on where we are when we look at the jewel. Any particular bit of truth is always related to other bits. The exact relations we see and use depends on us.
Second, there is a sense in which human knowledge is also relative to use. The acid test of whether particular people know something is whether they are able to use it in relevant situations. The person who claims to know something but who cannot apply an insight may well know the words without having really understood. Knowledge is thus always knowledge in relation to other truths and situations of possible use.
Finally, human knowledge is relative to time. Each of us can grow in knowledge, or forget, or cease to believe the truth.
The Sermon on the Mount and Human Flourishing: A Theological Commentary, by Jonathan Pennington (Grand Rapids: Baker Academic, 2018).I've been using this book as my main helper while preaching very slowly (because very occasionally) through the Sermon on the Mount....