Should A Christian Woman Wear A Bikini?
likewise also that women should adorn themselves in respectable apparel, with modesty and self-control, not with braided hair and gold or pearls or costly attire, but with what is proper for women who profess godliness—with good works. 1 Timothy 2:9–10 (ESV)
God also calls us to purity.
Let no one despise you for your youth, but set the believers an example in speech, in conduct, in love, in faith, in purity. 1 Timothy 4:12 (ESV)
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. A woman's appearance should not entice men to lust.
“You have heard that it was said, ‘You shall not commit adultery.’ But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart. Matthew 5:27–28 (ESV)
When a woman dresses in a way that causes men to look upon her body lustfully, she is inducing men to commit the sin of lust. This doesn't honor God.
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.
I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship. Romans 12:1 (ESV)
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 that is catching the eyes of men as a body being used for God’s glory.
Or course, wearing a bikini in a private location, like a fenced-in back yard, is acceptable, providing there is no visual access to the yard by the neighbors. A wife wearing a bikini around her husband exclusively for his enjoyment is acceptable. According to the verses above, Christian women have the responsibility not to put the males around them in a position where they might lust or have impure thoughts.
Portions in this post taken from Got Questions: Bible Questions Answered