Within the context of the first five verses of Romans 14, Paul is talking about Christians as servants of God and God as the master of all Christians.
Paul describes a situation where the faith of one Christian might allow him to eat only vegetables whereas the faith of another Christian might allow him to eat everything. Paul is saying it is wrong to look down upon a fellow Christian just because his faith means he thinks differently to another Christian.
We must not judge other Christians because each and every one of us has to answer to our own master, who is God.
In this illustration, Paul is pointing out that we are all God's servants and it is up to God to judge. Even the Christian with a weak faith will be enabled by God, his master, to stand.