How Can We Know if Something is Sinful?
In order to know if something is sinful, we need to employ our God-given reason, conform ourselves to the truth, and form our conscience in line with what God has revealed. First, we need to truly understand that we are capable of sin and that evil does exist. Many today deny that sin is possible, believe it or not. Likely, this is because we have lost touch with the reality of objectivity which is only found in God. Truth is not decided by the majority, it is as God made it. Our freedom, therefore, is not license to do whatever we like, it is the right to do what we ought.
Further, there are certain things which are rooted in the nature of mankind (called natural law). To discover these, man only needs to think clearly about himself and those around him. Murder, rape, and theft, for example, are clearly wrong. We do not need to be taught these things; as St. Paul says these are written on our hearts. Natural law exists because the all-knowing, all-powerful, and all-good God created the universe and the eternal law which governs it. Natural law is man’s participation in this eternal law.
Because of the fallen nature of man, following Adam and Eve’s first sin, we are inclined to sin, our intellect is darkened, and our free will is muddled. Nonetheless, we can begin to be conformed more and more with reality by striving for virtue in a life of grace. Apart from God’s grace, flowing through the Church, we are doomed. By God’s grace, living a life in the sacraments, we can come to know God more intimately, love Him more, and serve Him better.