Colin Kruse argues against mainstream interpretations of 1 John 3:19-22 as a digression. First, treating “heart” as a synonym for “conscience” is unprecedented in the NT [but there are examples in the OT, see 1 Sam. 24:5; 2 Sam. 24:6 – English verse numbering].
Second, there is no clear example in the NT where the Greek verb translated “set at rest” or “reassure” takes this meaning rather than its standard meaning “to persuade, convince” [other commentators point to Matt. 28:14].
Third, the opening “by this” in the other dozen instances in 1 John “carries forward the preceding discussion” which should lead as to expect a close relationship with the preceding [not merely a key word connection such as with “truth”].
“Bearing these three things in mind, and taking note of Deuteronomy 15:7-9 as the probable background to this passage, Court says that the interpretation offered by Sir Edwin Hoskyns  ought to be explored once more. Court argues
The demand for sacrificial charity has been made towards ‘a poor man, one of your brethren’ (Deut xv.7, cf. 1 John iii.17); but a base thought arises in the heart of a Christian which condemns the sacrifice demanded as unnecessary, and suggests that it can be avoided and that love can be maintained apart from a definite surrender of life or goods. The writer of the letter insists that this impulse, however natural, must be eradicated. The heart must be reasoned with and persuaded in the presence of God to make the sacrifice willingly. The demand of God is greater than the base and ignorant impulse of the human heart (cf. iv.4). Moreover, His knowledge is infinite, and no motion of the heart escapes his notice.
This approach provides a satisfactory resolution to the difficulties presented to the reader by verses 19-22, and makes way for an interpretation which takes full account of the integral nature of the whole section 3:11-24, and the place of verses 19-22 within it.”
We must persuade our hearts in the presence of God whenever they object to legitimate calls upon our generosity, when we are in fact in a position to respond.
“To assist his readers to persist in the necessary process of self-persuasion, the author provides them with a compelling reason for doing so…’because, if our heart condemns us, God is greater than our heart and knows all things’…God does not share in the meanness that is so often found in human hearts. His generosity is far greater, his compassion towards the needy much greater, than theirs. This fact should function as a reason for them to overcome the meanness of their own hearts and to seek to be like their God. When the author continues, ‘and he knows everything’, he is reminding his readers that any meanness of heart on their part will not go unnoticed by an omniscient God. As was the case in Deuteronomy 15:7-9, so too here, God knows what his people do, and judges them accordingly.
In summary, verses 19-20 function as a stern warning against that meanness of heart which objects to our expending material resources to meet the needs of fellow believers, and provide a foil for the positive reinforcement of generosity offered in verses 21-22.”
Colin G. Kruse, The Letters of John (Pillar New Testament Commentary; Grand Rapids: Eerdmans and Leicester: Apollos, 2000), 140-141.