Is it right for a member of the church to attend the regular meetings of the denominational church that their spouse, brother, sister, etc., is a member of? We must not let our human feelings or thoughts be our guide to answer this question. None of us are wise enough to answer this question without going to the Bible (Jer.10:23; Isa. 55:8-9). God's Word is the only place to find the answer. Our attitude must be one of accepting what God says in His Word, even if it means we must give up our own ideas.
The Bible is clear concerning those with whom we may not have fellowship: the Christian who does not walk after the apostolic traditions (2 Thess. 3:6); the Christian who causes division (Rom. 16:17); the bad, evil, corrupt Christian (1 Cor. 5). We may not have fellowship with unbelievers (2 Cor. 6:14-17), or any unfruitful works of darkness (Eph. 5:11).
God has always required His people to be separate (Lev. 20:24-26). God expected His people to be separated from the religious practices of other nations. He told them not to intermarry, realizing that to intermarry with idolaters would be to take the chance of being led away from the one true God unto false worship. (Solomon is a good example). This is one of many reasons why a Christian should marry a Christian. If it could happen to them, would it not be possible for it to happen to us?
God wanted His people separate, to have no part with idol worship; idol worship would be a false worship and false doctrine. But wouldn't that be true of any other false religious belief or system? Denominationalism is a false religious system. A religious denomination comes into existence only when a group originates by the preaching, believing, and obedience of some doctrine other than the true gospel of Christ. It teaches and practices error. What is said in the Bible concerning idolatry, and the fact that God's people should have no part in it, must also be said of denominationalism. Paul said, "Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them. For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple," (Rom. 16:17-18). They cannot be a part of our fellowship (2 Jno. 9-11).
However, there are some who will say, "If you attend our meeting, we will attend yours." The Bible says, "standfast in the faith ... be strong," (1 Cor. 16:13); "be strong in the Lord...,” (Eph. 6:10); "earnestly contend for the faith..." (Jude 3). You simply cannot compromise and contend at the same time. In every case when the Bible talks about false religious systems, the Christian responsibility is to have no fellowship with it, to earnestly contend for the faith and reprove the false religion. If someone attends our meeting and obeys the gospel, what has happened? Was it because you compromised and attended with them so that they would attend with you? NO! It was God's power, His Word, the gospel that converted the soul (Psa. 19:17; Rom. 1:16-17; Jas. 1:21). Never compromise; always be faithful to Christ.