Question: "Should a Christian woman wear a bikini?"
Answer: Whether or not to wear a bikini is a question many women struggle with, but for a Christian woman, the issue takes on additional implications. The Bible tells us that God calls women to modesty, which means to not draw attention to themselves: "I also want women to dress modestly, with decency and propriety, not with braided hair or gold or pearls or expensive clothes, but with good deeds, appropriate for women who profess to worship God" (1 Timothy 2:9-10). God also calls us to purity: "Don't let anyone look down on you because you are young, but set an example for the believers in speech, in life, in love, in faith and in purity" (1 Timothy 4:12). The question is whether or not wearing a bikini is consistent with modesty and purity.
Another issue to consider is that God calls all people to control their thought lives, so as women, we should help our brothers by dressing modestly: "You have heard that it was said, 'Do not commit adultery.' But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart" (Matthew 5:27-29). When we dress immodestly, it can lead them to commit the sin of lust, and this is displeasing to God. Please understand that each person is responsible for their own actions. In this instance, women are responsible for dressing modestly. Men are responsible for their thoughts.
A further consideration is that our bodies, like our minds and hearts, belong to God and are to be used for His glory, not our own. Romans 12:1 tells us, "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." When we offer our bodies to God as “living sacrifices,” we are saying, in effect, “My body is yours, Lord. Use it for your glory.” It’s hard to imagine a bikini-clad body being used for God’s glory.
Of course, wearing a bikini in a private location, like a fenced-in back yard, is probably acceptable, providing there is no visual access to the yard by the neighbors. According to the verses above, we have the responsibility not to put the males around us in a position that they might lust or have impure thoughts (see also Matthew 18:7).