Question: "What effects do conditions like autism, attachment disorder, ADHD, etc., have on the Christian life?"
Answer: When Adam sinned, death entered the world. God expelled Adam and Eve from the garden as they had exchanged their life in paradise for a curse. They were cursed, their bodies were cursed, and the world was cursed. Their bodies, once perfect, died a little every day. Their souls, once in tune with God, were now predisposed to rebel against Him. Thousands of years and hundreds of generations later, we are the heirs of that curse. Our bodies, despite medical science, are even more susceptible to disease than Adam and Eve's were. Our DNA, after generations of radiation and chemical exposure, is riddled with defects. And our society is so inundated with sin and our souls so easily trapped that our actions can alter our physiology in such a way that sin becomes even more natural.
God knows this. He knows that our sin and our physiology are sometimes connected. In Genesis 3:16, He explains the effect sin has on women: "Your desire shall be for your husband, and he shall rule over you." Sin had an impact on women such that future sin would be more prevalent. And God knows that sin—whether original sin, our own sin, or the sin of others—can change us in such a way that following God's standards becomes more difficult. The fallen state of the world has induced, directly or indirectly, many different conditions that weaken our understanding of God's plan for us. Brain abnormalities, such as those associated with autism, can make it hard to empathize with others and show love. The sin of others can cause attachment disorder, which damages the fellowship God designed us to have with others. And in-born physiology can make healthy sexual relationships difficult.
The first thing to realize about these conditions is that they are nothing compared to the power, love, and grace of God. Romans 8:38-39 says, “For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing, shall be able to separate us from the love of God, which is in Christ Jesus, our Lord." Nothing in us or outside of us, nothing we do and nothing done to us, can keep God from forgiving us and accepting us as His children. He knows our physical state and our difficulties. He knows our mental and emotional capacities. But we are all pre-conditioned to sin.
The second thing to remember is rather a double-edged sword: God knows our limitations, and we are still expected to obey. God knows our starting point as damaged people. His promises of hope and healing are just as applicable to someone with autism or a mental disorder as they are to anyone else. But, while we cannot contribute to Christ's work of salvation, we are expected to cooperate with God's work of healing in us. The principles of sanctification and dying to flesh still apply. We are still expected to consider God's plan for us above our own desires. That may mean continuing to take required medication, even if we don't feel like it. Or being diligent with therapy. And "healing" will not look like what the world expects, but God never intended us to conform to the world—He wants us conformed to the image of Jesus (Romans 8:29).
Everyone on this planet is predisposed to rebel against God and live a life of sin. Sometimes that predisposition has a name and medical significance, and sometimes it doesn't. But in God's eyes, there are only two types of people—those who have accepted Jesus as their savior, and those who haven't. Our physical limitations and proclivities may determine the challenges we will face in our attempt to obey God, but they need never separate us from God's love.