We can have a relative assurance that we are saved, should we die this moment. That is, if we truly believe we are in the state of grace. But since we do not know the future, we must never be so presumptious as to think that we might never fall from grace. We must always be vigilent. (Mt10:22; Rom 11:23;Phil. 2:12;1Cor 9:27;1Cor. 10:11-12;Gal5:4)
Salvation is a process. Jesus began the process by dying on the cross for us. He stored up an infinate abundance of graces for us to use during our lifetime. But we may reject them as well. His death on the cross made it possible to accept these graces. Without his redeeming work, we could never be saved. Now that they are there, the good that we do is sanctified by the good he did. By believing in him, we do as he asks.
It is not until we breath our last breath that the possibility of losing our salvation ceases. So Christ died to save us, we are being saved when we cooperate with his grace to do good and avoid evil, and we will be saved when he judges us worthy to enter heaven. Salvation is a past, present and future event…a process. He is the Judge, not us.