"The statement is trustworthy: For if we died with Him, we will also live with Him; If we endure, we will also reign with Him; If we deny Him, He will also deny us; If we are faithless, He remains faithful, for He cannot deny Himself" (2Tim.2:11-13). There is comfort here, as well as a warning. The comfort is in knowing that our God is always the same, He is not unpredictable from time to time, and therefore we can rely on Him completely. If He has promised something, He will keep it, and He does not say anything without meaning it. Every single plan He makes for us is for our welfare, even when we cannot figure them out, and He does not do anything that will harm us (Je.29:11). This passage says that He cannot deny Himself in the sense that He cannot take on any position other than being true to Himself, and He will be faithful to keep all His promises.
But there is also a warning here. We need to see there is a big difference between how we people tend to think and behave and how God does. For example, if a favourite friend or close relative of ours does anything wrong, we are tempted to bend the rules for their sake, even though we know that would be wrong. If we do that, we would not be faithful to ourselves or to God. We would be acting contrary to what we know to be right. But God is not like that. He cannot be unrighteous even for a moment in order to favour someone He is very proud of. We can see when we go through the Bible that He would show no partiality to anyone or stop being righteous for a moment for their sake. See how Moses lost the opportunity to take the people to the Promised Land because he dishonoured God at the last moment by disobeying Him.
So God says that if we deny Him, He would be forced to deny us. No matter how faithful we may have been and what all we have done for Him, we cannot expect Him to overlook our sins. Of course, there is forgiveness for us if we sin, but not if we imagine that God will forgive us and take sin lightly. God has to be faithful to being Himself, He cannot give up His righteousness or justice in order to accommodate us. Even when He forgives us, it is not as if He just overlooks our sins or takes them lightly as if they do not really matter. He has to be righteous and just too. He is able to forgive us only because our sins have already been punished through the death of Jesus and because we accept the position that what we deserve is judgment and that forgiveness is through His sheer grace.
If we have been thinking of God as if He was a man, let us stop that (Ps.50:21). Let us not expect that He will be after all kind and merciful and will not take our disobedience seriously. No, He cannot deny Himself, and He will always be faithful to Himself and in that way He will also be faithful to what He has revealed to us. Our love and gratitude towards Him should also be balanced with respect and awe towards His unchanging ways. There is really no contradiction here.