What does oneness refer to? Normally, the sutric system maintains that sentient beings are not buddhas but ordinary beings encumbered with defilement, who can transform their minds into the wisdom of the buddha only through Dharma practice over numerous lives. But the view set forth in the third turning of the wheel of Dharma, in particular that of tantra, is that ground and fruition are in fact one, that ground is fruition and fruition is ground. In other words, sentient beings are buddhas and buddhas are sentient beings; the two are one and the same.

~Depicted from GATEWAY TO VAJRAYANA PATH - The Completion Stage —the skillful means to realize emptiness