Romans 16:17

Now I beseech you, brethren, mark them that are causing the divisions and occasions of stumbling, contrary to the doctrine which ye learned: and turn away from them.

This, and through Romans 16:20, form an apostolic warning against false and divisive teachers whom Paul expected to trouble the unity and harmony of the church in Rome. Paul had evidently received remarkably full and accurate reports on what was happening in Rome, and there were many things for which he was no doubt thankful; but his experience had taught him that the crooked zeal of false teachers would eventually reach Rome, hence this powerful warning.

I beseech you ... is like the plea in Romans 12:1, and means "I beg of you, please."

Mark them ... means "identify them," "watch out for them," and "be on your guard against them." Whiteside commented thus:

Do not shut your eyes to what they are doing, nor make excuses for them, nor for any others who cause divisions and occasions of stumbling contrary to the gospel, but turn away from them. This means that the brethren should have no fellowship with them.[35]

Apparently, at the time Paul wrote, the leadership of the congregations in Rome had been able to preserve unity; and Paul's admonition here was given to strengthen their hands and warn them against heretical teachers already operating among the churches and sure to reach Rome in time.