Psalm 119:89 (NKJV)
Something that is unpredictable is something that unreliable, it is something you are uncertain of. For example, the weather can be unpredictable and it can be hard to say with any accuracy what the weather will really be like a day or two from today. If something is unpredictable, you cannot know exactly how it will act. On the other hand, if something is predictable, such as the sun rising in the East and setting in the West, then it is something that is a certainty and can be depended upon.
We all want things that are important for us to be dependable and reliable, but if they are unpredictable, they can be a problem. People themselves and what they do and say are often unpredictable. But one thing is certain, God and His Word are predictable. What God has said in His Word is something we can depend on and it is reliable. We don't have to be concerned that what God said in His Word will change from year to year or work differently in our life today than it did before.
We can accurately predict that 1 John 5:14-15 will work today just as it always has and that by praying according to God's Word, He will hear us and answer our prayer. We can predict and depend on the fact that those who call on the Name of the Lord will be saved (Romans 10:13) and that God is ready to perform His Word as He watches over it (Jeremiah 1:12). There are many things we can say about God's Word, but it is not unpredictable; God will always do what He has said in His Word!