With only six games remaining, Barca moved to 79 points and extended their lead to 11 points over second-placed Real Madrid, while Betis remain sixth on 49 points.
Christensen opened the scoring for Barcelona in the 14th minute with a point-blank header following a cross from Raphinha.
Barca's job was made easier by Betis defender Edgar Gonzalez, who came off the bench in the 12th minute to replace an injured Luiz Felipe but was sent off 21 minutes later after seeing two yellow cards for two bad tackles.
The locals made the most of their advantage before the break, scoring twice within three minutes thanks to strikes by Lewandowski and Raphinha.
Barca took their foot off the pedal after that, comfortable with seeing out the game, but managed to extend their lead thanks to an own-goal by defender Guido Rodriguez, who scored while trying to clear an Ansu Fati cross in the six-yard-box.
