As for the one who is weak in faith, welcome him, but not to quarrel over opinions. (ESV)
In the church there are people at different levels of maturity in their faith. This is just like any earthly family. You have different expectations of a toddler than you do a teenager. While it may be ok for a 6 month old to not be able to control their bladder you can be sure that there are different expectations of the eighteen year old.
Maturity. It is something that we are all working on in our walk with Christ. This was the case for those in the church whom Paul was writing to. Paul is speaking directly to those who were more mature. He knew that it was important for them to treat their more immature brothers and sisters in a proper and respectful way. They were to welcome the one who was weaker in faith.
God had put together a new race of people who belonged to him. Jews and Gentiles were now one in Christ Jesus. The Jews were set free from the ceremonial law of feasts, festivals, food restrictions and other requirements of the Law. The Gentiles were delivered from their bondage to worshiping false idols. They could now enjoy eating meat without the religious entanglement of idol worship.
Where there was a problem was that for new believers who had been Jews, they had a hard time abandoning some of the feasts and regulations. For the new believers who had previously worshiped idols it seemed heretical to be eating meat that was first offered to idols even though it was just meat, regardless of what was done to it. There was a necessity to love these more immature brothers.
How prone are you to argue with someone in the church who you feel does not know as much as you do in your understanding of the truth? In what way can you offer grace to a more "immature" believer?