Hebrews 5:11-14
11 About this we have much to say, and it is difficult to explain, for you have become sluggish in hearing.
12 Although you should be teachers by this time, you need to have someone teach you again the basic elements of the utterances of God. You need milk, (and) not solid food.
13 Everyone who lives on milk lacks experience of the word of righteousness, for he is a child.
14 But solid food is for the mature, for those whose faculties are trained by practice to discern good and evil.
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Here’s more on the idea of spiritual maturity. In this passage it is clear that it takes an effort of will to achieve spiritual growth. It is easy to "become sluggish in hearing" and mediocre in our faith. I know I’ve done that from time to time. It’s crazy that I can forget how joyful spiritual growth is, but sometimes I do, and let the world close in on my thinking too much.
We have to commit to growth, to actively seek it out through prayer, study of scripture, and the sacraments. We have to actively seek "experience in the word of righteousness" – it’s not going to happen on it’s own, and God is not going to force it on us. Note that solid food is "for those whose faculties are trained by practice to discern good and evil." We have to seek righteousness, rather than wait for it to find us. That’s a big difference.
