If this song had remained merely poetry, most Christians would have readily identified the humanism in it. But combined with a very pleasant melody and enough mention of God to be at least partly true, it "sold". I loved this song as a child, and it influenced my expectations of life.
Advertisers know that you can sell just about anything with a good tune.
May the Lord Jesus help us to "take every thought captive to the obedience of Christ", even if it comes in a beautiful wrapper.