And saw heaven opened - Acts 7:56. See the notes on Matthew 3:16. This language is derived from a common mode of speaking in the Hebrew Scriptures, as if the sky above us was a solid, vast expanse, and as if it were opened to present an opportunity for anything to descend. It is language that is highly figurative.
And a certain vessel - See the notes on Acts 9:15.
As it had been - It is important to mark this expression. The sacred writer does not say that Peter literally saw such an object descending; but he uses this as an imperfect description of the vision. It was not a literal descent of a vessel, but it was such a kind of representation to him, producing the same impression, and the same effect, as if such a vessel had descended.
Knit at the four corners - Bound, united, or tied. The corners were collected, as would be natural in putting anything into a great sheet.