keep the faith

keep believing in a cause, especially when there is resistance

fight the good fight

be devoted to a good cause even when others oppose you
—We’ve got to keep the faith and not lose hope. The only way to stop the spread of drugs in our city is for everyone to keep fighting the good fight.

In his first letter to Timothy, Paul writes that many people turn away from God because of their desire for wealth. He tells Timothy, though, that his path should be different:

But you, as a person dedicated to God, keep away from all that. Instead pursue righteousness, godliness, faithfulness, love, endurance, and gentleness. Compete well for the faith [Fight the good fight of faith] and lay hold of that eternal life you were called for and made your good confession for in the presence of many witnesses. (1 Timothy 6:11,12)

And in his second letter to Timothy, Paul, knowing that his life is nearing its end, writes,

For I am already being poured out as an offering, and the time for me to depart is at hand. I have competed well [fought a good fight]; I have finished the race; I have kept the faith! (2 Timothy 4:6,7)