The Doctrines of Grace.
Reformed theology emphasizes the doctrines of grace, best known by the acronym TULIP, though this does not correspond to the best possible names for the five doctrines.
T stands for total depravity. This does not mean that all persons are as bad as they could possibly be. It means rather that all human beings are affected by sin in every area of thought and conduct so that nothing that comes out of anyone apart from the regenerating grace of God can please God. As far as our relationships to God are concerned, we are all so ruined by sin that no one can properly understand either God or God’s ways. Nor do we seek God, unless He is first at work within us to lead us to do so
U stands for unconditional election. An emphasis on election bothers many people, but the problem they feel is not actually with election; it is with depravity. If sinners are as helpless in their depravity as the Bible says they are, unable to know and unwilling to seek God, then the only way they could possibly be saved is for God to take the initiative to change and save them. This is what election means. It is God choosing to save those who, apart from His sovereign choice and subsequent action, certainly would perish.
L stands for limited atonement. The name is potentially misleading, for it seems to suggest that reformed people want somehow to restrict the value of Christ’s death. This is not the case. The value of Jesus’ death is infinite. The question rather is what is the purpose of Christ’s death, and what He accomplished in it. Did Christ intend to make salvation no more than possible? Or did He actually save those for whom He died? Reformed theology stresses that Jesus actually atoned for the sins of those the Father had chosen. He actually propitiated the wrath of God toward His people by taking their judgment upon Himself, actually redeemed them, and actually reconciled those specific persons to God. A better name for “limited” atonement would be “particular” or “specific” redemption.
I stands for irresistible grace. Left to ourselves we resist the grace of God. But when God works in our hearts, regenerating us and creating a renewed will within, then what was undesirable before becomes highly desirable, and we run to Jesus just as previously we ran away from Him. Fallen sinners do resist God’s grace, but His regenerating grace is effectual. It overcomes sin and accomplishes God’s purpose.
P stands for perseverance of the saints. A better name might be “the perseverance of God with the saints,” but both ideas are actually involved. God perseveres with us, keeping us from falling away, as we would certainly do if He were not with us. But because He perseveres we also persevere. In fact, perseverance is the ultimate proof of election. We persevere because God preserves us from full and final falling away from Him.
Taken from “Reformed Theology” by James Montgomery Boyce