This is a very short parable, and speaks for itself quite clearly. In the wintertime, when trees lose their leaves, one cannot tell the difference between the trees that are dead and the trees that are alive. Likewise…
neither are the righteous manifest in this life, nor sinners, but they are alike; for this life is a winter to the righteous, and they do not manifest themselves, because they dwell with sinners… all are alike to another.
In the past, Hermas has not been shy about illustrating the difference in character between the righteous and the unrighteous; virtually all of the Mandates made this clear. And the high standard of ethics (high expectation of holiness among believers) has certainly been nothing to sneeze at. So what this similitude points out is that despite the drastic internal difference, the visible difference between believers and nonbelievers is pretty much zero.
The only hint of a reason offered for this phenomenon is that the righteous “dwell with sinners.” This makes perfect sense. Think about it: someone comes to true faith in Christ, goes through Baptism/Confirmation, receives the Holy Spirit, and all that jazz, but is that person now perfectly sinless? No! Life in this world still continues, and this world is still corrupted with sin, so the situation will still effect the believer. A great change of character should be occurring, and a great battle against sin is taken up, but universal struggle of good vs. evil continues, just as it does for everyone else.
This does not mean that believers can slack off, since we’re not going to look any different from nonbelievers. On the contrary, the invisible difference between dead and living trees in the winter is huge: the living trees are preparing to grow new buds and leaves, and the dead trees are continuing to rot. Those who are in Christ, then, should be growing in the fruits of the Spirit, so that when winter is over and Christ returns, the judgment will be favorable according to God’s calling.