"I was a lot more vigilant of losing my life after that. I became quite fearful, I became quite withdrawn," he testified.
Pistorius, who has been seen fingering a rosary during the trial and underlining sections of a religious book, said that Steenkamp "was a very good Christian."
"She would pray about my traveling and all of the small things in my life," he said through tears as he broke down crying again towards the end of the day's testimony.
"My religion is what's gotten me through this last year... my God is my refuge," he said.
Pistorius was preceded to the stand by Professor Jan Botha, a retired pathologist who testified for the defense.
Botha insisted the state pathologist's findings that Steenkamp had eaten no more than two hours before her death cannot be accepted. Pistorius has claimed that he and Steenkamp went to bed about 10 p.m. The prosecution's claim that Steenkamp had eaten about 11 p.m. would contradict Pistorius' timeline of events.