Sincerity is the passage from the cerebral to the cardiac, from the intellectual to the existential, from the partial to the total. The content of this transfer is the idea of Unity, and it is realized with the concurrence of the virtues, which for their part are so many modes or proofs of SINCERITY. (GTUFS: FaceA, Transcendence and Immanence in the Spiritual Economy of Islam) The root of all true SINCERITY is SINCERITY towards God, not towards our own good pleasure; this means that it is not enough to believe in God, but that all the consequences of belief must be drawn in our outer and inner comportment; and when we aspire to a perfection – since God is perfect and wants us to be perfect – we seek to have a semblance of it even before we realize it, and in order to realize it . . . The content of SINCERITY is our leaning towards God and our consequent adherence to the rules which this leaning imposes on us and not our nature pure and simple with all its shortcomings; to be sincere is not to be imperfect before men, but to be virtuous before God, and to enter accordingly into the mould of virtues as yet unassimilated, whatever men may think . . . Sincerity is the absence of falsehood in inward and outward behaviour; to lie is deliberately to mislead; one can lie to one’s neighbour, to oneself and to God. But a pious man who wraps his weakness in a veil of rectitude does not mean to lie and in virtue of that very fact he is not lying; he does not mean to manifest that which in fact he is, but he cannot help manifesting that which he wishes to be. And in the nature of things, he ends up by being perfectly truthful; for what we wish to be is, in a certain sense, what we are. (GTUFS: EsoterismPW, Sincerity: What it Is and What it Is Not)
Sincerity implies two initial concrete attitudes: abstention from what is contrary to truth, and accomplishment of what is in conformity to it; in other words, it is necessary to abstain from all that draws one away from the Sovereign Good – which coincides with the Real – and to accomplish all that brings us closer to it. This is why to the virtues of veracity and SINCERITY are added those of temperance and fervor, or of purity and vigilance, and also, even more fundamentally, those of humility and charity. (GTUFS: RootsHC, Virtue and Way)