Ah, that is a fruitless question. It is to same to ask: how can I be certain I won’t sin tomorrow? It is a total waste of time and energy. We are the most unreliable people. Save your energy rather and focus on making the next decision right instead.
Think however about the following truths:
– God loves us and He is doing absolutely everything right now to help us to be holy and saintly, to fulfill our calling by loving God and loving our neighbours.
– we are already justified, made into right relationship with Him when we were baptized, and when our sins were forgiven each time we went to confession; so if we have committed sin, go to confession to restore that relationship as soon as possible.
– every day, there is enough grace for us to make the right decisions to choose God; so focus on inviting God in the present moment as often as we can, He will not let us down if we hold on to Him.
– if we have failed, don’t be afraid to run to Him; He is for us and not against us, He has chosen us and will not forsake us; trust Him.
Living the above one day at a time, you will come to know you are always safe in Him, for you will come to love more and more what He is calling you to do. There is then nothing that can disturb or to frighten you.
There is nothing about ourselves that we can be certain of. But we can be certain that God is love and His mercy for us does not change. So long as we choose again and again to fix our gaze on Him, to follow Him, we can be certain that we won’t be led astray.