Our definitions of righteousness and sin are dependent upon the selection of a target. While you can choose to aim at anything, the ideal target is...the ideal target.
God—the ideal model of human character—is the ideal target. Jesus said:
Be ye therefore perfect, even as your Father which is in heaven is perfect. (Matthew 5:48)
Can anyone be better than perfect? In God, we find everything worth aiming for: his character, his situation, his joy, and his presence.
Sin means deviating in any way from your sincere understanding of ideal behavior. In other words, sin is deviating in any way from what God would do in your place.
There is an important interplay between the secular and religious views of ideal. The religious have an explicit idea of God, but shy away from overtly attempting to live according to it, deterred by the distance they feel between themselves and God. They fail to realize that their idea of God is not God himself, but merely their estimate of who and how he is. This estimate is much more reachable than God himself (and, as I will argue later, completely within the capability of every person to adhere to). The secular view of ideal tends to feature the kinds of practical, everyday applications that religionists are deservingly ribbed for overlooking, as the Lord and his followers have aptly and repeatedly pointed out (see, for example, Jesus’ parable of the Good Samaritan or his rebuke of the scribes and Pharisees in Matthew 5:20). However, the practicality of the secular ideal tends to be counterbalanced by lower expectations and less stretching than the explicit idea of God.