This is a very difficult, interesting, and challenging question. Some would say that since believers in Christ are "new creations" with "all things made new" (2 Corinthians 5:17), the sin and consequences of divorce are washed away, allowing a person who was divorced before becoming a believer to be remarried. Others would say that while the sin of the divorce was atoned for by Christ, the consequences of the sin are not, and therefore a person who was divorced before becoming a believer cannot remarry.
Making this question even more difficult is the fact that there are varying viewpoints on whether Christians can remarry. Please read the following articles:
When the Bible talks about marriage, it does not speak only to Christians/believers getting married. The biblical principles on marriage are universal. If an unsaved man and woman get married, they are just as married in God’s eyes as a Christian man and woman who get married. They are still one flesh (Genesis 2:24). God still hates divorce (Malachi 2:16). God has still joined them together, and He does not want them to be separated (Matthew 19). While all of our sin—past, present, and future—is forgiven when we are saved, salvation does not wipe away all the consequences of the sins we committed before we came to faith in Christ or the sins we continue to commit. Jesus paid the penalty for our sin, but sin still has real and negative consequences. It is our belief, then, that, whether pre-salvation or post-salvation, if the divorce was for unbiblical reasons, there are no grounds for remarriage.
However, as the articles listed above indicate, we believe in the exception clause. If a divorce occurred as a result of unrepentant, continual adultery, we believe the innocent party can remarry. This is equally true if the innocent party was a believer or unbeliever when the divorce occurred. So, the answer to this question would depend on the circumstances of the divorce. It is our contention that whether the divorce occurred before or after salvation is not the ultimate deciding factor. Whatever viewpoint a person takes on this particular issue, it is important to understand that salvation does not free us or excuse us from all the foolish and sinful decisions we made before coming to faith in Christ.