The biggest opponent we face is not the one that stands across from us on the mat, whose hand we shake prior to the first whistle at the start of a match. It's not even the one we see in mirror, although that person can sometimes be a challenge. No, the Bible says in Ephesians 6:12, "For we do not against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places." Once we recognize who the real enemy is and what his tactics are (see John 10:10), then we can prepare for the fight and do so FROM victory through the shed blood of Jesus Christ on the cross and the power of the Holy Spirit.
"...the one who is in you is greater than the one who is in the world." - 1 John 4:4