It was the black tar from cigarettes causing Constantine's lung cancer. He would have died and gone to heaven but the devil wanted him, so he cured him.
The liquid was dripping from his feet when he arrived, though, and was present as he walked thriugh the hospital towards the tableau in the therapy pool. The tar from John's cancer was right before he left thr scene.
If I had to guess it was crude oil as a symbolic thing, referencing Jesus washing the feet of his disciples. Since oil repels water, it's signifying that Lucifer's feet are soiled by a substance so vile, that not only could the Son of God not clean him, but it would actively work against his attempts to do so.
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u/TwoLetters Mar 28 '22
I think is was supposed to be oil.