From studying Matthew 12:36 seems to me that Jesus was referring to all the “empty words” (idle and careless) that were ever spoken from both believers and unbelievers. Unbelievers will be judged for their words at the Great White Throne Judgement when it mentions in Rev. 20:12 that certain “books were opened.” Since the Matthew 12 verse is also for believers, when will this take place?
From Matt. 12:25 to the end of the chapter, Jesus was talking directly to the Pharisees who had just accused Him of performing miracles by the power of Satan. This is confirmed in Matt. 12:34. We know that unbelievers will be judged at the Great White Throne (Rev. 20:11-15) but there is no such judgment for believers. All our sins were forgiven at the cross and washed away in the blood of Jesus. From God’s perspective we’re as righteous as He is.
It’s important to follow this thought to its conclusion, because that’s where Jesus spoke abut believers. In Matt. 12:37 He said, “For by your words you will be acquitted and by your words you will be condemned”
If we say words of repentance such as, “Lord Jesus I know I’m a sinner in need of a Savior, please forgive my sins and be my Savior” our words will acquit us. The Holy Spirit will come to dwell n us, preventing us from saying words that condemn us like the Pharisees did.