Is it wrong to exclude a believer from fellowship because of doctrinal differences?

Is sectarianism sin? I am In an assembly of so called “open brethren” who invite “Reverend” gentlemen from the sects, also a sister, to address them. Is this right?.

Sectarianism wherever found, is a work of the flesh (Galatians 5:20). To whom but God Himself should confession be made? The beginnings of it were seen at Corinth—”I am of Paul, and I of Apollos and I of Cephas; and I of Christ” (1 Corinthians  1:12).

In the first three cases men were attaching themselves to human leaders, instead of to Christ alone; in the last case, while attaching themselves to Christ, they were detaching from Him true members of His Body and pretending to belong to Him in some peculiar sense, in which others did not. This is also sectarianism against which we must watch. The whole idea of dividing up the Body of Christ is condemned by the Holy Spirit (see 1 Corinthians 3:3-4 ). It is condoned to-day and we are told that the different sects are only regiments in God’s army, as though regiments recruited from one another and were sometimes found fighting against one another. The only true Christian position is to know no other centre than Christ, and to recognise all believers as equally belonging to Him, though lamenting the broken-up condition of things around.

No doubt there are men truly gifted of Christ, who occupy sectarian positions, but that does not make those positions right, and to invite such to teach the Saints a complete anomaly and certainly not calculated to “build these up on their most holy faith,” but rather to break down the land-marks between truth and error and lead the Lord’s people back on to sectarian ground. This is especially dangerous when the invited brethren are men who were once in the simple assemblies themselves, but have become popular preachers or even “Reverends,” wearing all the insignia of their declension. They are often strongly opposed to the truth they once professed. It is not for us to judge their conscience, nor should we put a salve to their conscience by inviting them to teach or preach, thus approving of their position, or be present, when they officiate, as is apparently the case in some places. As for inviting a sister to teach the gathered saints, it is so clearly contrary to our Lord’s express prohibition, through His apostle: “I suffer not a woman to teach, nor to usurp authority* over the man, but to be in silence” (1 Timothy 2:12; see also 1 Corinthians 14:34-35), that comment is needless.