Romans 15:27No child would survive into adulthood without parents to help out. Whether those are natural parents, foster parents, older relatives, or a sad conglomeration of reluctant caretakers, babies are not capable of obtaining their own food and clothing and other basic needs for the first several years. Having received so much, it is both natural and appropriate to give back to those who provide for us when they become needy and we are the capable ones. For us as Christians, it is also a commandment with the promise of a blessing.
We give back to our spiritual parents for the same reasons. We have been blessed so we acknowledge a certain responsibility to return the blessing. The return might look quite different, as Paul describes a material gift responding to a spiritual inheritance. The return might look more like a passing forward or a sharing in mission. Honoring our parents when we are older might include verbal respect, helping them maintain their independence, physically caring for them, financially providing for them, contributing to organizations or causes they were (/are) passionate about, encouraging their relationships with grandchildren, caring for the needs of other young children. Honoring our spiritual parents can look similar, depending on our relationship with those who have helped us become the Christians we are today. We won't generally care for the physical needs of authors whose books have transformed our worldview. But we might contribute to what they are passionate about and care for other "children" who also need that transformation. We might share that book with others, act on its call to action, or communicate its message to others we encounter. We might pray for them. We also "give back" to those who have blessed us when we "give forward" by praying and caring for people who are in the position we used to be in. If you don't know anyone...well, 1) Make a change so you are meeting people who need the prayers and spiritual or physical care you have to offer and 2) Ask other Christians around you to help connect you to people in need. I know many more people in need than I can provide for, from individuals who just need prayers to persons who need friends to reach out to them to people who could benefit from buckets of love yet will probably never be in a position to return the investment poured into their lives. We might not be able to give back to Paul directly for the spiritual blessings we receive when we read his letters to early Christians, but we honor him in a very real way when we evangelize other "gentiles" to a relationship with Christ Jesus.