This is coming from Bede's Historia Ecclesiastica Gentis Anglorum, and it was a part of the Christianization of the Anglo-Saxons.

According to Bede's history, the Isle of Wight was one of the last remaining Anglo-Saxon kingdoms that was pagan. The King Cædwalla conquered the island from the king Arwald and supposedly put all of the inhabitants to the sword because of their pagan ways. Save two royal youths who were baptized before being executed.

Now it is really quite impossible to quantify these claims. Bede is writing after the fact, and as a Christian monk is not exactly an unbiased source. Of course by the same token, his claims cannot be dismissed outright. He was not writing that long after these events, they would have been in living memory of some people, whether Bede ever met such a person and interviewed them is another matter.

This is the relevant passage

After Cædwalla had possessed himself of the kingdom of the Gewissae, he also took the Isle of Wight, which till then was entirely given over to idolatry, and by cruel slaughter endeavoured to destroy all the inhabitants thereof, and to place in their stead people from his own province; having bound himself by a vow, though he was not yet, as is reported, regenerated in Christ, to give the fourth part of the land, and of the booty, to our Lord, if he took the island, which he performed by giving the same for our Lord to the use of Bishop Wilfred, who happened at the time to have accidentally come thither out of his own nation. The measure of that island, according to the computation of the English, is of twelve hundred families, and accordingly the bishop had given him land of three hundred families.

Now, this seems to have been a pretty small kingdom. 1200 families isn't an significant number of people, but its hardly a massive population.

Now the passage seems to indicate that either there was a large influx of Christian families to replace the slaughtered pagans or that not all the pagans were slaughtered and they were converted. Unfortunately there isn't really a whole lot to go on outside of Bede for this.

So take that information for what you will.

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Sources,

Bede's, Historia Ecclesiastica Gentis Anglorum from Fordham Sourcebooks (if you want the link to the entire work I'll provide it)