We should say what Scripture says. The two most notable on this topic are the following:

1Jn 2:19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us.

Mat 7:22 On that day many will say to me, 'Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?'
Mat 7:23 And then will I declare to them, 'I never knew you; depart from me, you workers of lawlessness.'

False professors who eventually fall away are described in Jesus' parables as tares among wheat and bad fish among good. Notice that in the parable of the sower, it's only the seed sown in "good soil" that produced grain. Seed planted in the other soils appeared to be viable, but all died-off before producing "fruit."

Jim

Jud 1:3 . . . contend for the faith that was once for all delivered to the saints.