If the Bible commands us to be steadfast, as it does in 1 Corinthians 15:58, then is it a sin to be unsteady? If God desires us to be immovable, same verse as above, is everyone who is erratic in disobedience? In other words, is it a sin to be weak?
It could be, depending on the type of weakness and reason for weakness.
Our physical bodies are in a state of weakness, certainly compared to our bodies when raised in power (1 Corinthians 15:43), and that corruption isn’t moral. Getting old isn’t sinful, being born blind isn’t sinful, many other diseases and sicknesses aren’t due to disobedience; think Job. But, if your body is weak because of your gluttony, or because of your drunkenness, or because you’re addicted to laziness, your weakness is at least a symptom of sin.
There are other types of weakness than physical, and even the exhortation to steadfastness is not a call to join a gym for sake of better physical fitness. We can be weak in faith, weak in heart, weak in conviction and commitment. Is that kind of weakness sin?
Again, it could be. Are you weak in heart because you’re a baby Christian who’s learning the faith, or because you’re an old (not necessarily mature) Christian who doesn’t want to do the work of paying attention to the teaching of Scripture? Are you weak in faith because it’s being tested, and you feel weak and cry to God to help you believe more? Or are you weak in faith because you’ve decided that drinking milk is easier than constantly distinguishing between good and evil (Hebrews 5:13-14)?
What type of weakness do you have and why are you weak? What do you do with the weakness? Recognizing weakness while obeying is one thing, claiming weakness as an excuse to disobey is another. There is a weakness that leads to even greater loss.
The one who conquers will have this heritage, and I will be his God and he will be my son. But as for the cowardly, the faithless, the detestable, as for the murderers, the sexually immoral, sorcerers, idolators, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death. (Revelation 21:7-8)