Question: "What is consequentialist ethics / consequentialism?"
Answer: Consequentialism is a theory of normative ethics. It holds that an act is only moral or ethical if it results in a good conclusion. This is in contrast to deontology, which teaches morality is based on duty; virtue ethics, which holds that morality is based on a good character; and ethical relativism, which asserts morality is based on whatever you want it to be based on.
Consequentialism is a slippery theory and has led to a great many arguments about the specifics. After all, a person can "aim" his actions with the intent of causing a specific result, but the outcome is out of his hands, for the most part. Are we to believe that every moral action must be followed by a good outcome in order to be considered truly “moral”? What if someone fights nobly for a good cause, but fails in the end? Are the goodness of the cause and the nobility of the fight negated by a bad outcome?
If morality is based on “a good consequence,” then we must ask, "What is ‘good’?" Which is better, to gain pleasure or avoid harm? What is more important, filling a need or filling a preference? The secular worldview can give no clear answer.
To try to narrow down the definition of “good,” philosophers also discuss the question "good for whom?" A leaky roof is a burden for a homeowner but good for the roofer. A college acceptance letter means another student was rejected. Would it be better to improve the welfare of the acting agent or a bystander? Or society at large?
Some consequentialists admit that the intention of the acting agent may have something to do with the morality of the act. But then we must determine who has the authority to judge whether the intention was appropriately considered—the acting agent? a neutral third party? a system of laws? "Actual" consequentialists dismiss the entire discussion of the “almost” consequentialists and insist that morality is based solely on the actual effect; "almost" only counts in horseshoes and hand grenades.
The Bible speaks of consequentialism, but not by name, and not in the way that secular philosophy considers. The Bible says people ought to act morally; that is, they should follow God's law and the guiding of His Spirit in their hearts. And the Bible also teaches a certain end effect of morality.
Consequentialism in God's economy comes in the form of telos. Telos means "purpose," and it informs all of God's laws. His Word is not arbitrary. The entire history of mankind is filled with the story of God's purposes for us.
"This book of the law shall not depart from your mouth, but you shall meditate on it day and night, so that you may be careful to do according to all that is written in it; for then you will make your way prosperous, and then you will have success" (Joshua 1:8).
“‘For I know the plans that I have for you,’ declares the LORD, ‘plans for welfare and not for calamity to give you a future and a hope’" (Jeremiah 29:11).
"Those who love Your law have great peace, and nothing causes them to stumble" (Psalm 119:165).
Morality does have a consequence, and it should inform our decision to perform moral acts. But the consequences are not some nebulous, unknowable, uncontrollable happenstance. God created morality for a purpose: "Nevertheless the righteous will hold to his way, and he who has clean hands will grow stronger and stronger" (Job 17:9).