You remember how Peter has told us that Paul’s epistles deal with these matters [MLJ is referring here to 2 Peter 3:16-17], ‘in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction; ye therefore, beloved, seeing ye know these things before, beware lest ye also, being led away with the error of the wicked, fall from your own steadfastness’. Instability: the child is unstable, changeable, easily depressed, easily frightened and easily discouraged. There is hardly any necessity to comment upon these things. But transfer all that to the spiritual realm. The Christian person who is always becoming panicky is obviously a babe in Christ. We are set in this world, and there are things happening round and about us – are we easily discouraged, are we easily depressed? Do these things that happen immediately make us question the whole of our faith and the love of God to us? Are we unstable and changeable? If so, we are but babes in Christ. Nothing is so characteristic of that stage of life and development as just that.
– Martyn Lloyd-Jones, Expository Sermons on 2 Peter, p. 247.