46 Comm. on Gen. 4:2f,7.
DAILY FAMILY WORSHIP
"The rite of sacrificing more fully confirms this. Because it proves that they had been accustomed to the worship of God" (at the very least in the outward sense).
Of course, as to the very important matter of motive, Cain and Abel were poles apart. Nevertheless, continued Calvin: "The life of Cain…was – in appearance – very well regulated…. He cultivated the duties of piety toward God, and sought a maintenance for himself and his, by honest and just labour – as became [or behooved] a provident and sober father of a family [cf. Genesis 4:17]….