Nowhere is "imagination" in this verse. Quite the opposite. God is saying faith isn't wishing or imagining. It is confident expectation and assurance of what God has said in His Word (and notice He starts with creation by His Word), and certainly not things you want to get or accomplish.
These are the words to describe "faith". The believers mentioned in the chapter didn't imagine anything (how can you imagine what you can't see or conceive of?). Imagination is fleshly, fallible, and temporal. It is precisely what isn't faith. It's an attempt to walk by what one sees with one's mind.
Act 4:25 Who by the mouth of thy servant David hast said, Why did the heathen rage, and the people imagine vain things?
1Ti 4:15 Take pains with these things; be absorbed in them, so that your progress will be evident to all.
So, Warren is twisting Scripture by using it and inserting into the text what isn't there, and then claiming it's "essential to living by faith".
He's preached about "dreaming" (you dream, "God" dreams, etc),and having "visions" (vision casting, "God's" visions for us) etc...all of which are not only unbiblical, because he's a false teacher and lost so he has no understanding of spiritual things (1Cor. 2:14), but they are indeed mystical.
God's Word is enough to live by, after all that is the point of Heb.11; the old saints trusted in God and His Word even though they couldn't see it. These men and women were tortured, persecuted, and killed for their faith--hardly a time to be walking around imagining things when stones are hitting your head or you're being sawed in two. Rather, keeping Scripture foremost in one's mind--the Word of God which is living, active, perfect, powerful, and true, is what anchors our minds. It is Scripture that we set our minds on, not vain imaginings. Scripture is "the mind of Christ" (1 Cor. 2:16). His Word is our bedrock foundation, our anchor, unmoveable, unchanging, established Truth. After all Scripture comes from the mouth of God. The point is to walk by what God says, not by what we think or wish things to be or what we wish to have or whatever.
Yet John Piper said about Rick Warren:
I saw a man with character and with simple, childlike dependence on Christ.An unbelievable communicator with incredible application. Be encouraged and relax with who you are, and give it all to Jesus while learning all you can from Rick.
Col 2:3 in whom are hidden all the treasures of wisdom and knowledge.
Col 2:4 I say this in order that no one may delude you with plausible arguments.
Col 2:6 Therefore, as you received Christ Jesus the Lord, so walk in him,
Col 2:7 rooted and built up in him and established in the faith, just as you were taught, abounding in thanksgiving.
Col 2:8 See to it that no one takes you captive by philosophy and empty deceit, according to human tradition, according to the elemental spirits of the world, and not according to Christ.
1Co 2:11 For who knows a person's thoughts except the spirit of that person, which is in him? So also no one comprehends the thoughts of God except the Spirit of God.
1Co 2:12 Now we have received not the spirit of the world, but the Spirit who is from God, that we might understand the things freely given us by God.
1Co 2:13 And we impart this in words not taught by human wisdom but taught by the Spirit, interpreting spiritual truths to those who are spiritual.