It is very easy for a believer to be ignorant of his true spiritual condition, if he does not live before God's face. This is clear from the rebukes that the Lord gave to the leaders of the seven churches in Revelation. To the messenger (elder) of the church in Laodicea, he said, "You do not know that you are wretched, miserable, poor, blind and naked” (Rev. 3:17).
God allows different situations to arise in our lives that will expose what lies hidden in our hearts. Through the years we have stored many unpleasant memories in our hearts as a result of the difficult experiences we have had with different people. They lie hidden at the bottom of our hearts - and we imagine that our hearts are clean. Then God allows some small thing to happen, that stirs up all this rotten stuff and brings them all into our minds. That is the time we must cleanse ourselves and forgive the people involved and decide to love them. If we don't use such an opportunity to cleanse these things from our hearts, they will sink to the bottom again after the turmoil is over, and remain in our hearts. We can then imagine that all is well. But it isn't so. Another small event can bring them all up to our mind again. So, we must cleanse ourselves every time something surfaces.
We see in the case of the elder brother of the prodigal son, how he had a wrong attitude towards his younger brother. Yet this surfaced only when his brother came back and a feast was made for him. Then we see how he accused his brother with accusations that he imagined, without verifying whether his statements were true or not (for example, that his younger brother had "wasted his money on harlots” Luke 15:30). When we don't have a good relationship with someone, we will always believe the worst things about him.
The father told his older son "All that I have is yours”. Instead of being occupied with what his father gave him, the elder brother was occupied with his own accomplishments: "I never disobeyed any of your commands. I have served you all these years”. He was also occupied with the shortcomings of his brother, "This son of yours has wasted your money” (Luke 15:29-32). Like that father, God also tells us, "All that I have is yours”. Everything that is in Jesus is ours - all His purity, all His goodness, all His patience, all His humility etc.
The lesson we have to learn from this story is just this: Be occupied always with the riches of God's grace - and not with your own accomplishments, or with the failures of your fellow-believers.
God allows different situations to arise in our lives that will expose what lies hidden in our hearts - and we must cleanse ourselves every time something surfaces.