As followers of Christ, we might just answer this question by saying “nothing.” We might think that we shouldn’t boast about anything at all–that we should simply be humble servants of the Lord and our friends and neighbors.
God through Jeremiah says that there are definitely things we shouldn’t boast about but that there are indeed things that we should boast about: “This is what the Lord says: ‘Let not the wise boast of their wisdom or the strong boast of their strength or the rich boast of their riches, but let the one who boasts boast about this: that they have the understanding to know me, that I am the Lord, who exercises kindness, justice and righteousness on earth, for in these I delight,’ declares the Lord.” (Jeremiah 9:23=24, NIV)
If we have to boast, we are not to boast about ourselves. We are to boast about the Lord whom we follow–about his kindness, justice, and righteousness.
Careful, though. If we boast about having the understanding to know God, that can come across as boasting about ourselves, not God. It’s not always what we say that’s a problem; it’s how we say it. Also, if we’re going to boast about God and his nature, it’s important that God’s kindness, justice, and righteousness can be seen in us.