Q. What do you say to those polygamist men who claim that they are following in the footsteps of Abraham and have no earthly authority above them but themselves?
A. Bunkum. For one thing, we are under the New Covenant in which every true believer is placed within a local assembly, congregation or church where is he part of a Body which is presided over by a Pastor and Elders to whom he is subject and accountable. There is no such thing as a solo-Christian/Messianic and there is no such thing as a solo-Patriarchal (Polygamist) Christian/Messianic either.
For another, Abraham was not his own earthly master - he was subject to the Order of Melchizedek and to the High Priest of the same name (Shem) based in the city of Salem to whom he paid his tithes (ma'aser).
Whilst it is perfectly true that words like 'organisation', 'congregation' and 'fellowship' did not exist in patriarchal times it is completely wrong to suggest that relational groups without heads and authority structures were absent. The evidence - quite apart from common sense - defies such an assertion.
A semi-romantic image was cultivated quite early on in the patriarchal Christian polygamy movement in the United States of men with wives and wealth roaming like nomads on camels doing prety much as they please that has more in common with modern hedonistic ideas than Biblical ones. There are many men - far too many - who consider themselves above any kind of law or recognised biblical fellowship structure who are self-appointed pastors of their own mini-kingdoms. Most of them are too proud and haughty to be governed.
The basic unit of the Body of Christ has always been - and will always be - the local assembly or fellowship with the proper New Testament-mandated officers and ministries: apostles, prophets, pastors, elders, deacons, teachers, etc.. Needless to say this is not liked by many ambitious and lawless men who would rather stand apart and do things their way as lords of the manor. Their excuses are numerous - the one you cite is one of them. The other is that all the churches are in 'apostacy' and they have been called to 'stand apart' from them. Bunkum again. Yes, there is apostacy, and on a large scale everywhere, but do you think Elohim (God) would be so careless as to not ensure that there are congregations around which are acceptable in His sight? True, there are very few that accept polygamists, but if polygamists are sincere about their professions of faith, they should relocate so that they can fellowship with other Christian/Messianic polygamists and place themselves under pastoral headship and accountability. In the meantime, for those not in a position to move, there are cyberfellowships and pastorates which, though far from the ideal, are better than nothing at all.
Polygamy does not suddenly give a man the liberty to throw out the local congregation or to redefine it as a bunch of unaccountable friendly people without a delineation of Body functions such as is found in an assembly. Biblical freedom is nowhere defined as coming out from under authority but placing oneself under it. Biblical freedom is to be removed from the stain of sin by faith in the blood of Christ and is proved by our voluntary obedience to all the commandments of New Covenant Torah.
Finally, there are those ultra-Protestants who think the 'Bible' is enough, and that all they have to be 'under' is a Bible. Bunkum again. The Bible is the manual of the Church (Messianic Community)! The Bible is the guiderail of the local assembly not the other way round.
Steer clear of the headless nomadic patriarchs!