In reality, many would do or try the best they can in order to achieve the best result. However, how many can achieve as one expects, I suspect not that many. So there would be a lot of disappointments and frustrations. But I learned from the positive thinking books taught me that we have to keep going and focusing on what we want to get until we get it. Many successful people are based on this principle and are successful. Being Christians we are able to get the help from the Holy Spirit and we can perform the best if we depend on the Holy Spirit instead of our effect. We can still fail but we must have the faith to keep doing what we can do without stopping.
In Galatians, it’s said that God continually supplies us with His Spirit and works powerfully through us as a free gift—not because we’ve done anything or deserve it. (Galatians 3:6) So also Abraham “believed God, and it was credited to him as righteousness.” Abraham put himself in God’s hands despite his failures, and God used him to bring salvation to the world through his lineage.
There’s no justification for behaving badly. Jesus has asked us to follow Him in obedience, and He supplies the means to do so. A hard and unrepentant heart will always hinder His purposes for us, but His ability to use us doesn’t depend on a whole series of good behavior. It’s based solely on God’s willingness to work through us as we are: saved and growing by grace. You don’t have to work for His grace.