It seems that Luther had quite a bee in his bonnet (if he wore a bonnet) about Christians living a Christ-centered life. The first three of his theses all focused on repentance and how it should effect our lives both inside and out.
The Catholic church (at the time as well as now) placed a heavy focus something called penance, it is a theological error stemming from a mistranslation of repentance which kind of stuck, as well as an incorrect belief that the Grace of God is not sufficient to bring us back into relationship with God. At its core, penance means that we have to do something apart from being repentant to get right with God.
Luther calls us to move past this faulty thinking and embrace the heart of Jesus' mission, message and life. We embrace it by cutting through the rubbish and living lives that are repentant. That means being in constant examination of our actions and motives and when we don't act in a way God would like, saying sorry to Him and show a change in attitude that really reflects that we are sorry.
Luthers (translated) words were:
1. When our Lord and Master, Jesus Christ, said "Repent", He called for the entire life of believers to be one of repentance.
2. The word cannot be properly understood as referring to the sacrament of penance, i.e. confession and satisfaction, as administered by the clergy.
3. Yet its meaning is not restricted to repentance in one's heart; for such repentance is null unless it produces outward signs in various mortifications of the flesh.
These we may effectively paraphrase into:
1. One's whole life should reflect repentance and a heart for the kingdom of God.
2. The life of the kingdom cannot be administered by clergy, it's up to the individual.
3. This inner change of heart must have an outward expression also.
The evidence of repentance, indeed the evidence of a Christian life is one that inside and out is loving God by their thoughts and actions. Being constantly on our guard examining our motives and actions isn't really as onerous as it sounds, when you practise a life of repentance you start to become more in-tune with how God want's you to live and it starts to become your default response in situations. In other words, the more you think like Jesus, you become more like Jesus.