Is there one thing, above all others, that a person can do that will lead to a spiritual life and leave behind the religious Christian life?

Well, start by looking at what sin looked like at the very beginning. “You certainly won’t die!” the snake told the woman. God knows that when you eat it your eyes will be opened. You’ll be like God, knowing good and evil. The woman saw that the tree had fruit that was good to eat, nice to look at, and desirable for making someone wise. So she took some of the fruit and ate it. She also gave some to her husband, who was with her, and he ate it. Then their eyes were opened, and they both realized that they were naked” (Genesis 3:4-7a God’s Word).

Sin resulted from a lie told and a lie believed. Then the belly took over because the fruit was good to eat, tasty looking. It was also pretty. And disobeying somehow would make one wise, if you believe the lies. And when the rebellion against God began, the first evidence of being apart from Creator was not wisdom, but lust.

The Good Samaritan by Steven Sawyer

So, the true disciple does not follow the belly nor the eye as a reliable guide on their own. Both can be deceived and can lead us astray.

That being said, Jesus clearly taught what the spiritual life consist of. “ ‘Love the Lord your God with all your heart, with all your soul, and with all your mind [Deuteronomy 6:5].’ This is the greatest and most important commandment. The second is like it: ‘Love your neighbor as you love yourself [Leviticus 19:18b].’ All of Moses’ Teachings and the Prophets depend on these two commandments” (Matthew 22:37-40;  see also Mark 12:30-31 and Luke 10:27 God’s Word, Old Testament references added).

Love God without reserve and without distraction and without split loyalty to anything. And love your neighbour, meaning act with respect and compassion as God does. Feed, clothe and nourish them in sickness and imprisonment (Matthew 25:31-46).

If you do this you won’t have any spare time to get religious.