I don't like seeing the flag of any country inside a church building or outside a church building.
I don't think there should be a "Christian" flag either. God's elect, those He chose before the world and gave to His Son to be their Surety and Redeemer, and who God sovereignly places under the preaching of the finished work of Christ on behalf of His elect, and who are sovereignly regenerated by the Holy Spirit, are to worship God the Father, God the Son, God the Holy Spirit, and not some piece of cloth with some symbol on it.
From the pulpit, the gospel of Christ alone should be preached, and not worldly politics, regardless of what the political subject is, be it cultural issues, or economic issues, or foreign policy issues.
What did the apostle Paul preach? "For I determined not to know any thing among you, save Jesus Christ, and him crucified." 1 Corinthians 2:2
Holding this or that political position will not save anyone's soul. Placing this or that flag in or outside of a church building will not save anyone's soul.
I am thankful that God sovereignly brings His elect under the hearing of the gospel of Christ and teaches them to stay away from the worldly, false gospels that are so prevalent today.
"Yet, here is mercy, that the Lord saves whom he will save, and that we are saved by free grace, and free grace alone, through the blood and righteousness of the Son of God. "He of God is made unto us wisdom and righteousness and sanctification and redemption;" so that if we have him we have everything, and if we have him not we have nothing. Where these things are felt they will cause exercise of soul, with many prayers and supplications to the God of all our mercies; and all this will strip and empty us of that light, superficial, and flimsy profession which seems so current in our day." - J.C. Philpot