"Wherefore I will not be negligent to put you always in remembrance of these things, though ye know them, and be established in present truth." 2 Peter 1:12
As you already know from experience, some truths are eternal and stay the same throughout all generations and some truths which seem to be true at the time are found to be erroneous in due time. Think of it this way. If you never changed out what you previously believed for what you believe today, you would still have the mind of a child. We do change out what we thought was true to what we know is true today. And that is the message in this verse. Peter, a Jew, was talking to Jews and Gentiles. He himself had to change what he thought was true before Jesus to everything he learned and is continuing to learn after he met Jesus. This was true of all the Apostles and it is true for you also.
Peter wanted to edify those whom he was speaking to by reminding them of the new found truths that were introduced by Jesus, even though he knew that most of the people he was writing to knew what he was talking about, but he also wanted to remind them that non-eternal truths change and/or what we once thought was a truth may not have been a truth at all and we need to operate in present truth.
If you try to fight today's battles with yesterday's armor, you will not be successful. Every time you read your Bible it is likely that you will learn something that when added to the truths that have already been confirmed, advance your knowledge and understanding of God's economy.
God is pouring out His Spirit on mankind and sending a message 24/7 to all who will hear His voice. If you are listening to God, you will be receiving truth that can and will alter your current understanding of life. That is present truth. Operating in present truth means that if you are practicing what you already know to be the truth (Philippians 3:16), you are operating in the will of God. The consequences of not operating in present truth is revealed in churches that are living in the past, no growth, no life, no joy and no power.