"Freedom" here is an idiom meaning the ability to live as one wants. AT: "God has chosen you to be his people so that you can live as you truly want" (See: [[rc://en/ta/man/translate/figs-idiom]]).
The relationship between the opportunity and the sinful nature can be stated more clearly. AT: "an opportunity for you to behave according to your sinful nature" (See: [[rc://en/ta/man/translate/figs-explicit]])
Possible meanings are 1) "you can state the whole law in just one commandment, which is this" or 2) "by obeying one commandment, you obey all the commandments, and that one commandment is this."