My husband slapped me one time and he said he wouldn't do it again. Can I believe him?

It is possible that this violent action does not happen again. However, these excuses habitually occur as part of a "cycle of violence" often seen in the behaviour of an abuser: violent episodes are interspersed by periods which are more or less calm, during which the abuser makes excuses and swears it won't happen again. There is therefore no guarantee that your husband will not do it again.