Ralston tapped in the rebound of Kenny Mansally's shot in the 15th minute. Cristman headed in a rebound of Jeff Larentowicz's drive in the 32nd minute.
Reis, who had 10 shutouts last season, made a spectacular diving right-hand save on Houston's Stuart Holden in the 60th minute. On the ensuing corner kick, Nyassi cleared a drive by Ed Robinson off the goal line.
Cristman, a second-year striker who started 14 matches last season, was a late addition to New England's starting lineup as Revolution career scoring leader Taylor Twellman (91 goals) was scratched with a knee injury.
New England outshot Houston (0-1) 8-3 during the first half and put relentless pressure on Dynamo goalie Pat Onstad.