According to the Bible, does sin give Satan 'Legal Access' to your life in any sense?

This answer is coming from the viewpoint of a Conservative Independent Baptist that believes that the Bible is the ultimate guide to truth.
It also assumes that the phrase "legal access" in your question means "legal claim", and is intended to mean the official definition of the phrase.

claim 1) v. to make a demand for money, for property, or for enforcement of a right provided by law. 2) n. the making of a demand (assert a claim) for money due, for property, for damages or for enforcement of a right. If such a demand is not honored, it may result in a lawsuit. In order to enforce a right against a government agency (ranging for damages from a negligent bus driver to a shortage in payroll) a claim must be filed first. If rejected or ignored by the government, it is lawsuit time.

claim (Right), noun beneficial interest, contingent interrst, due, equitable interest, expectancy, heritage, interest, legacy, ownership, privilege, share, stake, title, vested interest Associated concepts: claim of ownership, claim of right, claim of title

It also assumed that you are referring to sin in the life of a believer after salvation (based on the context). This answer is not to contradict the core doctrinal tenets of original sin, or of unbelievers/unsaved persons belonging to Satan. As covered elsewhere, we adhere to those teachings, and believe that if a person is not saved, he or she is lost already. (John 3:18 and others)

I've heard this statement as well, and your question prompted me to see if I could look into it myself. The main issue that I have with the teaching is that in all my Bible studies, I've never heard a Scriptural basis for such a claim.

I managed to find some teachings on the Internet on the subject, but these are also without Scriptural backing. Examples, all of which make the claim that our sin gives Satan a legal right to us, or that Satan fights a legal battle. I'll skip including quotes on each because they all essentially make that same statement:

• http://www.greatbiblestudy.com/legalrights.php
• http://www.simplychurch.com/2010/10/kingdom-skills-causes-of-emotional-problems-5-when-satan-has-a-legal-right-.html
• http://yourlastresort.net/index.cfm?PageID=5606

The list goes on.

The closest I came to a site that had actual Scriptural basis cited Revelation 12:10, KJV

And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night.

This is often explained as Satan being our accuser in God's court of judgment. This is how I understand the passage as well, but the idea of Satan being our accuser in a "court" is not the same as giving Satan a legal claim on our lives.

The meaning of these statements seem to indicate that by giving Satan a legal claim, we make ourselves his and give him full power to possess, harm, and destroy us. The accuser in the final judgment is accusing us in the trial for our final destination. These are two very different things.

The statement that sin gives Satan a legal right may sound right, but without Scripture to back it up, I see no reason to accept it as truth. I base this on the lack of Scriptural support and the following:

• 1 John 4:1, KJV which says Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
• The authors of the New Testament went into great detail on doctrinal issues. Paul, in the many books he authored, went to great pains to ensure that the truths that are important were understood. If this were an important, doctrinal issue, it would be evident in Scripture.

This is not to say that there are no truths outside of Scripture. Scripture never states that 1+1=2, yet it's true. However, as a doctrinal issue, this one sounds good, but fails the test of Biblical support.