Righteousness Before God
You are going to serve somebody. Jesus says not to do righteousness (dikaiosune) in order to be seen by men, because if you do then God will give you nothing for it (v. 1). But if you serve Him in secret these three ways—giving, praying, and fasting—He will reward you openly (v. 18). In both scenarios there is an audience and a reward, and this is necessarily the case: we will live for an ultimate purpose in front of an audience and desire a reward, even if we reduce that purpose and reward to our own appetites. The scribes and Pharisees loved theatrical righteousness, to be seen doing good things by the crowd. But Jesus wants His people to do righteousness to please God, and therefore to be greatly rewarded.