The age of consent in Indiana is, in fact, 16.
This is the relevant law. However:
1) As her pastor, Schaap was in a "position of authority or substantial influence over the victim," meaning the 16-year age of consent did not apply.
2) He was specifically convicted of violating the Mann Act, which prohibits taking women under 18 across state lines for immoral purposes. This is a federal law, and again, Indiana law does not apply.
All other things being equal, if she was 18, she might still very well be a victim of an exploitive authority figure. Schaap would then be guilty of both adultery and professional misconduct, which would be more than enough for him to be thrown to the curb forthwith and never employed by a church again.
However, it's doubtful that his conduct would have been criminal. Not everything grossly immoral is illegal.