On the straight and narrow

Keeping on the straight and narrow is about resisting the temptation to do something wrong, or making sure you don't fall into bad habits.The saying is a slight mistranslation from the words of Jesus as they appeared in the old King James version of the Bible (first published in 1611). There, Jesus says, "strait is the gate and narrow the way" which leads to life. "Strait" is an old English word which means "narrow", but it's become "straight" in the modern saying. Here's the full saying of Jesus from the King James Bible...

Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat: Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.

Matthew 7:13-14