A forward thinking man was advocating that men and women should be free to decide their actions uninhibited by social moorings or moral compunction. This annoyed all his friends married and unmarried. He got married, and wisdom dawned on him. He confessed to his friends, "I am ashamed of having thought of free action by men and women before my marriage. It looks barbaric now that I am married." He was sincere in his convictions before and now. But it was the sincerity of vital feeling, not rational thinking. The phrase Sincerity of Purpose has a wider implication, wider than the thinking Mind. It speaks of Truths not locally valid for a context or person, but eternally valid.
Sincerity of Purpose is of the Spirit. We are in our vital emotions. Rarely in issues that do not bother us personally, rational minds speak rationally. A head of the family had some ayurvedic preparation meant to sustain health. It was brought as powder to be taken with rice as if it is sambar. Everyone revolted at its smell. They were all browbeaten into compliance. Tyranny succeeded over the revolt. Finally he sat down to that 'elixir'. He found it abominable. He got up without eating it, saying, "I did not know it was this bad." Nobody had been permitted to open his mouth to say, "you taste it first". Nor had he conceived of it earlier.
The Sufi teacher who asked the mother of the sugar loving child to come later had the humility NOT to preach what he had not practised. He gave up eating the sugar before the mother returned and that was why his advice had an effect.
People who start a lucky venture often voluntarily promise 50 % of the profits to their faith. When the profits come, they conveniently forget it. If some busybody reminds them of the promise, the usual reply is, "I am not that generous." The FACT as far as they are concerned is they were sincere in their promise when it was made. Now too they are sincere. There is no hypocrisy on their part. Only that they are living in the living Present. It is the Spirit that lives in all three times, past, present, and future.