GOD'S WORD® Translation
They left us. However, they were never really part of us. If they had been, they would have stayed with us. But by leaving they made it clear that none of them were part of us.

American Standard Version
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 they might be made manifest that they all are not of us.

Berean Study Bible
They went out from us, but they did not belong to us. For if they had belonged to us, they would have remained with us. But their departure made it clear that none of them belonged to us.

Douay-Rheims Bible
They went out from us, but they were not of us. For if they had been of us, they would no doubt have remained with us; but that they may be manifest, that they are not all of us.

English Revised Version
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 they might be made manifest how that they all are not of us.

King James Bible
They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.

World English Bible
They went out from us, but they didn't belong to us; for if they had belonged to us, they would have continued with us. But they left, that they might be revealed that none of them belong to us.

Young's Literal Translation
out of us they went forth, but they were not of us, for if they had been of us, they would have remained with us; but -- that they might be manifested that they are not all of us.

1 John 2:19 Additional Translations ...