The works of the righteous would be mortal sins if they would not be feared as mortal sins by the righteous themselves out of pious fear of God.- Martin Luther May 1518
Luke 17:7 "Will any one of you who has a servant plowing or keeping sheep say to him when he has come in from the field, 'Come at once and recline at table'? 8 Will he not rather say to him, 'Prepare supper for me, and dress properly, and serve me while I eat and drink, and afterward you will eat and drink'? 9 Does he thank the servant because he did what was commanded? 10 So you also, when you have done all that you were commanded, say, 'We are unworthy servants; we have only done what was our duty.'" ESV
Find God's purpose for your life.
Fill up that heavenly bank account with good deeds.
Contribute to and draw on the treasury of merit.
These are all pleas from various traditions which place a huge emphasis on what one does for God. All of them promise (threaten) a measuring of good works when we meet up with Jesus. The word of God does not give a list of good deeds to do except perhaps helping the poor and needy which is what God requires of all people.
So the laundry list comes out of the mouths of these preachers advising you to give more, volunteer in the church more, attend more worship and bible studies. All of those are good things with which to be involved. But what those preachers do not tell you is that obeying the speed limit is also a good work in the eyes of God. Going to work on time and getting the work completed is a good work. Washing the dishes for the family is also viewed as a duty that God expects. These also can and are being done by believers and unbelievers alike. So what is the difference?
Attitude and motivation is the difference. A Christian will do these things with love for Jesus and with fruit of the Holy Spirit. An unbeliever will not because he or she does not love Jesus nor has the Holy Spirit bearing fruit. In Jesus God's children robed in His righteousness do all sorts of things which please God.
So why would those deeds be mortal sins as referred to by Dr Luther? If they are presented to God as if He needed them. God certainly as creator and sustainer of the universe does not need our good works. He is the originator of good works and gift giving. So how can we hold these up to God and tell Him this is for you because you needed it? Another reason they would be sin is due to mixed motivation. Humans are self centered by nature. The good done for society, employer and family are partially motivated by hope of reward and fear of punishment. That is the way the worldly system operates to keep the sinners from abusing other sinners and the work gets done. The sin becomes faith endangering due manipulation of God as if He owes us for our paltry efforts.
Jesus serves us totally devoid of self interest. God, being God, does not need creatures to fulfill Himself. He does these things for that is His nature. He tabernacled among us and endured the temptation, bitter suffering, humiliating death and glorious resurrection for you. He was not a greater God because of it nor is He more worthy of praise for doing it. Those things were His from eternity past. Now what work can you or I do to match that?
Glory be to the Father and to the † Son and to the Holy Spirit.