See below. Just because he is a boy from church, does not me he does not have an std. Also, you greatly enhance your risk for pregnancy. Use protection.
After you marry him. Unprotected sex is almost never safe from transmission of illnesses and /or risk of pregnancy when it's between a fertile male and female couple. It is much safer in a committed, monogamous relationship in which the two of you would embrace a pregnancy and be prepared to raise a child. Even in the confines of a monogamous relationship, previously acquired infections can be transmitted.
Safe from what? Any sexual contact you have exposes you to anyone that person ever had sex with in the past. Unprotected sex exposes you to the chance of pregnancy & STD's that are incurable or can make you sterile. The only safe sex occurs in a monogamous relationship between two uninfected people.