In Judaism, in the mind and the culture of the day in which James lived and wrote, truth was the basis of the law. Reading James 5.12 it would be very easy to hyper-moralise and say something like,
You need to be so careful to tell the truth
so you don’t fall under condemnation.
Be a better, more truthful you.
The law-minded person would simply see this as a law to keep.
What about when we look at this through the lens of Jesus? We see that our character is lacking so we feel the need to make oaths, we feel we need to substantiate our claims by saying something like ‘I swear to God I will…I swear I didn’t…’
The fact that James is saying this at all is proof it was happening, isn’t it, you don’t teach about situations that don’t exist, do you? If we are honest, this is still happening; we feel the need to make stronger our own words, our own claims than with a simple yes or no.
Jesus, on other hand, didn’t.
Jesus spoke only truth in love. We fail, He succeeds. You will never be that more truthful you until you are a born again believer in Jesus with the Spirit of God living in you. How is that possible?
Faith in Jesus makes this possible.
Truth, Biblical truth, Christian truth, Jesus-emanating truth is authenticity, Divine reality. John in particular in his account of the life, ministry, death, resurrection of Jesus uses this word ‘truth‘ to describe the fact that this reality, this authentic, Divine reality is actually possible for you and me as humans, but in our natural state, it’s out of reach. We are out of touch, out of step with it, but, through the revelation of the Word of God, the incarnation and work of and faith in the Word of God, it is reachable.
So, with James writing about yes be yes and no be no, in our natural state, sadly, nobody is ever going to be able to get to that standard. No motivational speech, no self help, no self improvement, no amount of burden that you put on yourself will ever get you there. There is only one way to be able to live this life, there is only one way to practice truthfulness as God Himself is truthful and that is through the authenticity, the Divine revelation, the Word become flesh, through Jesus.
Everything that we want, everything that we need to live a life of truth, true, actual, authentic, real truth, is available to us in Jesus, and nowhere else.
So, simply, your life of truth has to be rooted in, has to stem from, to grow out of, Jesus.