When seeking the answer to issues that are not expressly mentioned in the Bible, we can and must "reason" from Scripture. This means we can find principles we can apply to every situation in our lives by comparing Scripture with Scripture in context. Obviously, gastric bypass surgery was not an issue when the Bible was written. Therefore, this is an area that we would call "doubtful," because there is no clear "you shall or you shall not" for us to turn to. It is not expressly a "sin" to have elective surgery. The question is, what is the purpose and will the end result have the effect of bringing us closer to God and glorifying Him in our lives? In other words, how will this surgery affect our personal relationship with God?
There are several principles that we could cite that might help us to come to a decision. However, one thing is very important. None of us can tell another born-again believer what he should or should not do with his own body where Scripture does not specifically address the issue. This is between the believer and the Lord. We can study, search for principles and present them, but it is up to the individual believer how he applies those principles to his own life. Believers stand before the Lord alone. Born-again believers are commanded and may indeed choose to come alongside to comfort, counsel, and encourage one another, but we cannot make others’ choices for them.
One important issue here is walking by faith. If one is not fully convinced in one’s own mind that an avenue of action is right and is not sinful, then as believers we must err on the side of righteousness. Romans 14:23 tells us that "whatever is not of faith is sin." In other words, if we have the conviction in our own minds that what we are doing is sin, then to us it is sin. Therefore, when it comes to those things that we do not have clear commandments on, we must make the choice to obey by faith first. However, let us be clear, we are not referring to inordinate guilt for a course of action. Rather, it is the "still small voice" of wisdom—the gift of the Spirit—that is important and not the accusations of our enemy who seeks to place us in bondage. If we are walking by faith and we are in fellowship with the Lord, then we do not have to worry about being in God’s will—God’s will always finds us. And the answer will be clear and not muddied with guilt. The Holy Spirit never accuses us; rather, He convinces and convicts us with the result that we choose to walk in obedience (John 16:12-14). The strident voice of accusation is always from our enemy (Revelation 12:10).
As born-again believers, we are free to do whatever we want within the boundaries of Scripture. The Apostle Paul makes that very clear in two passages in 1 Corinthians. "’Everything is permissible for me’—but not everything is beneficial. ’Everything is permissible for me’—but I will not be mastered by anything" (1 Corinthians 6:12). What the apostle is teaching in this passage is that grace makes us free in Christ. However, we must make the choice to discern what is "expedient" or convenient, what "edifies" or builds us up spiritually, and not to allow what we are doing to control our lives. The only thing that should control the lives of the born-again believer is the fruit of the Spirit (Galatians 4:22-23). In other words, just because we can do something does not mean that we should. Again, the important criterion must be what is beneficial to our own personal relationship with the Lord.
As born-again believers, we are to be faithful stewards of every aspect of our lives (1 Corinthians 4:2). That includes our bodies. We are to be temperate in all things and we are to do "all things" to bring glory to God (1 Corinthians 10:31). Excessive weight is a health issue, to be sure. However, the heart attitude that fuels the desire to lose weight must be examined. Being a faithful steward includes telling ourselves the truth. We can try to deceive ourselves but when we get down to the bottom of our issues with weight, the reality is that, unless we have a genuine medical problem that causes excess weight, we gain or lose weight by our own choices. Every choice we make has a consequence. Being a faithful steward means doing all things temperately and allowing the Holy Spirit, not food, to control our lives.
Given these principles, we can come to some pertinent truths. First, we are not to walk by feelings but by faith. Second, we are to be temperate in all things and strive to do those things that will bring glory to God in our lives. Third, we are to be faithful stewards of all that God has given us. The choice to undergo stomach surgery is a serious matter. It is elective surgery, and any surgery comes with its own set of risks. When one makes the choice of whether or not to go ahead with this course of action, he must search his own heart using biblical principles, do some investigation, talk truthfully with doctors, and trust God to reveal the right choice. "Therefore, I urge you, brothers, in view of God’s mercy, to offer your bodies as living sacrifices, holy and pleasing to God—this is your spiritual act of worship. Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God’s will is—his good, pleasing and perfect will" (Romans 12:1-2).