The Belgic confession of faith,

Article XXIX The Marks of the True Church, and Wherein it Differs from the False Church

We believe that we ought diligently and circumspectly to discern from the Word of God which is the true Church, since all sects which are in the world assume to themselves the name of the Church.

But we speak not here of hypocrites, who are mixed in the Church with the good, yet are not of the Church, though externally in it; but we say that the body and communion of the true Church must be distinguished from all sects that call themselves the Church. The marks by which the true Church is known are these: If the pure doctrine of the gospel is preached therein; if it maintains the pure administration of the sacraments (baptism & The Lord's Supper) as instituted by Christ; if church discipline is exercised in chastening of sin; in short, if all things are managed according to the pure Word of God, all things contrary thereto rejected, and Jesus Christ acknowledged as the only Head of the Church. Hereby the true Church may certainly be known, from which no man has a right to separate himself.

With respect to those who are members of the Church, they may be known by the marks of Christians; namely, by faith, and when, having received Jesus Christ the only Saviour, they avoid sin, follow after righteousness, love the true God and their neighbour, neither turn aside to the right or left, and crucify the flesh with the works thereof. But this is not to be understood as if there did not remain in them great infirmities; but they fight against them through the Spirit all the days of their life, continually taking their refuge in the blood, death, passion, and obedience of our Lord Jesus Christ, in whom they have remission of sins, through faith in Him.

As for the false Church, it ascribes more power and authority to itself and its ordinances than to the Word of God, and will not submit itself to the yoke of Christ. Neither does it administer the sacraments as appointed by Christ in His Word, but adds to and takes from them, as it thinks proper; it relies more upon men than upon Christ; and persecutes those who live holily according to the Word of God and rebuke it for its errors, covetousness, and idolatry.

These two Churches are easily known and distinguished from each other.