In order to warn his fellow believers about Jews who would try to get them to follow the old laws, Paul gives his own testimony about when he persecuted believers.
Here "these things" refers to Paul's teachings. You can add this alternate translation to the end of the previous sentence. Alternate translation: "because these teachings will protect you from those who teach what is not true" (See: [[rc://en/ta/man/translate/figs-explicit]])