I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.
How glorious is the thought and the truth that “Christ is in me!” The very Son of God, the one by whom everything was made that is made (John 1:3) makes his abode in the very hearts of His people. The way He does that is through the Holy Spirit, whom He has sent. (John 16:7) For we know that Christ personally sits at the right hand of the Father in Heaven. (Heb. 1:3)
When Christ lives in us through His Spirit, He gives us the strength to accomplish every task that He gives us to do. There are certainly things that He requires of us, and it is only by the simple fact that He lives in us that we are able to do His will.
Yet, caution should be taken when considering this great truth not to fall into one of the many heresies promoted when considering scriptures such as Gal. 2:20 out of context. J.C. Ryle writes in his introduction to his work, Holiness:
They will see there that two centuries ago the wildest heresies arose out of an extravagant teaching of this very doctrine of the "indwelling of Christ" in believers. They will find that Saltmarsh, and Dell, and Towne, and other false teachers, against whom good Samuel Rutherford contended, began with strange notions of "Christ in us," and then proceeded to build on the doctrine antinomianism, and fanaticism of the worst description and vilest tendency. They maintained that the separate, personal life of the believer was so completely gone, that it was Christ living in him who repented, and believed, and acted! The root of this huge error was a forced and unscriptural interpretation of such texts as "I live: yet not I, but Christ lives in me." (Galatians 2:20) And the natural result of it was that many of the unhappy followers of this school came to the comfortable conclusion that believers were not responsible, whatever they might do! Believers, forsooth, were dead and buried; and only Christ lived in them, and undertook everything for them! The ultimate consequence was, that some thought they might sit still in a carnal security, their personal accountableness being entirely gone, and might commit any kind of sin without fear! Let us never forget that truth, distorted and exaggerated, can become the mother of the most dangerous heresies. When we speak of "Christ being in us," let us take care to explain what we mean. I fear some neglect this in the present day.
So we see that when Mr. Ryle wrote this in the 1800’s, this was a very real issue that the church faced. I can only imagine that such heresies have proliferated today. According to Webster’s 1828 dictionary, an Antinomian is:
One of a sect who maintain, that, under the gospel dispensation, the law is of no use or obligation; or who hold doctrines which supersede the necessity of good works and a virtuous life. This sect originated with John Agricola about the year 1538.
While we are not justified by the law, the law gives us the foundation for living a life of holiness and morality. Be very careful, fellow Christian, to remember that Jesus Christ kept all the law on our behalf. What’s more, to the extent that we do indeed “die to the flesh” and allow Him to live through us, we will “keep the law” by allowing His Spirit to work through us. The scripture reminds us that if we walk in the Spirit we will not fulfill the lusts of the flesh. (Gal 5:16)
For additional reading on this subject, I recommend the book, Holiness, by J.C. Ryle.