The middle form of the verb, which has a reflexive force. It is when an act returns back in its effect on oneself: see Note e, Heb. 1.3. Paul was to act thus in Acts 22.16; not to baptise himself, that would be active; but 'get baptised.' They passed through the sea and so got baptised. There was no action of course of a baptiser here, hence the middle voice.