Sometimes when I hear this, it makes me cringe. The Word of God is written and our primary means of absorbing it is through reading. I have on my mind about it but it was said better below by someone else.
Christians should be readers. We should read and meditate on the Bible, of course, but we should also read theology. Good theology systematizes and explains the Bible in ways we would be pressed to come up with on our own. Few of us are a Jonathan Edwards, John Owen, J. I. Packer or John Piper, and we would be wise to learn from them.
Most of us know we should read the Bible and theology. But what about other subjects, like literature, history, biography, science, and culture? And what about books by non-Christians? I think we should read widely, and yes, that includes reading non-Christians. John Calvin thought so too:
Therefore, in reading [non-Christian authors], the admirable light of truth displayed in them should remind us, that the human mind, however much fallen and perverted from its original integrity, is still adorned and invested with admirable gifts from its Creator. If we reflect that the Spirit of God is the only fountain of truth, we will be careful, as we would avoid offering insult to him, not to reject or condemn truth wherever it appears. In despising the gifts, we insult the Giver…. Nay, we cannot read the writings of the ancients on these subjects without the highest admiration; an admiration which their excellence will not allow us to withhold. But shall we deem anything to be noble and praiseworthy, without tracing it to the hand of God? (Institutes II.ii.15-16)
God has set up the world so that even non-Christians can find truth. I’ve learned truth from Christians and non-Christians. We can’t expect non-Christians to have sound theology, but they are some of the best authors in other subjects. If we reject their Spirit-given insights because they are non-Christians we, as Calvin says, “insult the Giver.”