3. A sincere footing is a sending forth of good deeds. Here it means that for the believers is felicity as has been earlier decreed by God and written in the Preserved Tablet for their sending forth of good deeds and believing in Muhammad (ﷺ) who guided them to the path that leads to Paradise. Safely treasuring up for themselves a fine foundation and noble position for the future, so that they may get a firm hold on the real life - life of the Hereafter.
4. They could only rationalize the bewildering effect of the Message of Islam by saying that it bewitches people.