For those verses given, here are the notes from the DRV:
1 “Try the spirits”… Viz., by examining whether their teaching be agreeable to the rule of the Catholic faith, and the doctrine of the church. For as he says, (ver. 6,) He that knoweth God, heareth us [the pastors of the church]. By this we know the spirit of truth, and the spirit of error.
2 “Every spirit which confesseth”… Not that the confession of this point of faith alone, is, at all times, and in all cases, sufficient; but that with relation to that time, and for that part of the Christian doctrine, which was then particularly to be confessed, taught, and maintained against the heretics of those days, this was the most proper token, by which the true teachers might be distinguished form the false.
3 “That dissolveth Jesus”… Viz., either by denying his humanity, or his divinity.