[3]. “A sincere footing” refers to the sending forth of good deeds. In this verse, it means that for the believers is a state of felicity already decreed by God and inscribed in the Preserved Tablet — a reward for the good they have sent ahead and for their faith in Muhammad (ﷺ), who guided them to the path leading to Paradise. They are thus securing for themselves a noble foundation and a firm standing for the life to come — the true, eternal life of the Hereafter.
This was revealed to address people’s astonishment that a human messenger — Muhammad (ﷺ) — could receive divine revelation. The disbelievers, however, rejected this truth and dismissed it as sorcery, out of arrogance and denial.
[4]. They could only rationalize the bewildering effect of the Message of Islam by saying that it bewitches people.