This is the argument made by the apostle Paul to the Colossians. They were being overwhelmed with false doctrines and philosophies concerning their standing before God. Some Jews were wanting to judge them according to the Law of Moses, others were insisting the pathway to righteousness was through extreme asceticism or even the worship of angels (Col. 2:8-23). Paul rejects all of these errors and reinforces their standing before God in Christ alone. "For in Him dwells all the fullness of the Godhead bodily; and you are complete in Him, who is the head of all principality and power" (Col. 2:9-10). They did not need anything other than Christ to be complete before God.
Sometimes I hear brethren say complimentary things about those of other faiths. They admire the zeal of the Mormons and Jehovah Witnesses in their door-knocking efforts. We are cautioned not to condemn all Muslims, because the non-radical ones are sincere, peace-loving, and are said to be "more devoted" to their religious beliefs than many Christians. The Seventh Day Adventists are admired for keeping the Sabbath Day, the Pentecostals are admired for their zeal, the Buddhists are admired for striving after self-enlightenment and peace, and the Amish are admired for living without modern conveniences. The list can go on.
While we can admire an individual's conviction and adherence to their beliefs, there is NOTHING in Mormonism, Islam, the Watchtower Society, or any other religion, society, or philosophy that can make us a better Christian. Nothing! We are complete in Christ. The word that is translated "complete" in our text means to be full. Brother Walton Weaver makes the following comment regarding Paul's teaching: "His point is that it is only in Christ that we find the satisfaction of every spiritual want, and because of this we have no need to go elsewhere in search for something additional or supplemental" (Truth Commentary on Philippians and Colossians, page 431).
We need to be careful that we do not begin to look to false religious practices, pop-psychology, self-improvement fads, and business models as a means of becoming a better Christian or improving the work of the local church. Everything that we need to be complete is supplied in Christ.
"All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work" (2 Tim. 3:16-17).