10 In the earliest times, “the Holy Ghost fell upon them that believed: and they spoke with tongues,” which they had not learned, “as the Spirit gave them utterance.” These were signs adapted to the time. For there behooved to be that betokening of the Holy Spirit in all tongues, to show that the Gospel of God was to run through all tongues over the whole earth. That thing was done for a betokening, and it passed away. In the laying on of hands now, that persons may receive the Holy Ghost, do we look that they should speak with tongues? Or when we laid the hand on these infants, did each one of you look to see whether they would speak with tongues, and, when he saw that they did not speak with tongues, was any of you so wrong-minded as to say, These have not received the Holy Ghost; for, had they received, they would speak with tongues as was the case in those times? If then the witness of the presence of the Holy Ghost be not now given through these miracles, by what is it given, by what does one get to know that he has received the Holy Ghost? Let him question his own heart. If he love his brother the Spirit of God dwells in him. Let him see, let him prove himself before the eyes of God, let him see whether there be in him the love of peace and unity, the love of the Church that is spread over the whole earth. Let him not rest only in his loving the brother whom he has before his eyes, for we have many brethren whom we do not see, and in the unity of the Spirit we are joined to them. What marvel that they are not with us? We are in one body, we have one Head, in heaven. Brethren, our two eyes do not see each other; as one may say, they do not know each other. But in the charity of the bodily frame do they not know each other? For, to show you that in the charity which knits them together they do know each other; when both eyes are open, the right may not rest on some object, on which the left shall not rest likewise. Direct the glance of the right eye without the other, if you can. Together they meet in one object, together they are directed to one object: their aim is one, their places diverse. If then all who with you love God have one aim with you, heed not that in the body you are separated in place; the eyesight of the heart you have alike fixed on the light of truth. Then if you would know that you have received the Spirit, question your heart: lest haply you have the sacrament, and have not the virtue of the sacrament. Question your heart. If love of your brethren be there, set your mind at rest. There cannot be love without the Spirit of God: since Paul cries, “The love of God is shed abroad in your hearts by the Holy Spirit which is given unto us.”
11. “Beloved, believe not every spirit.” Because he had said, “In this we know that He abides in us, by the Spirit which He has given us.” But how this same Spirit is known, mark this: “Beloved, believe not every spirit, but prove the spirits whether they be from God.” And who is he that proves the spirits? A hard matter has he put to us, my brethren! It is well for us that he should tell us himself how we are to discern them. He is about to tell us: fear not: but first see; mark: see that hereby is expressed the very thing that vain heretics taunt us withal. Mark, see what he says, “Beloved, believe not every spirit, but prove the spirits whether they be from God.” The Holy Spirit is spoken of in the Gospel by the name of water; where the Lord “cried and said, If any man thirst, let him come unto me, and drink. He that believes in me, out of his belly shall flow rivers of living water.” But the evangelist has expounded of what He said this: for he goes on to say, “But this spoke He of the Spirit, which they that believed on Him should receive.” Wherefore did not the Lord baptize many? But what says he? “For the Holy Ghost was not yet given; because that Jesus was not yet glorified.” Then seeing those had baptism, and had not yet received the Holy Ghost, whom on the day of Pentecost the Lord sent from heaven, the glorifying of the Lord was first waited for, so that the Spirit might be given. Even before He was glorified, and before He sent the Spirit, He yet invited men to prepare themselves for the receiving of the water of which He said, “Whoso thirsts, let him come and drink;” and, “He that believes in me, out of his belly shall flow rivers of living water.” What means, “Rivers of living water”? What is that water? Let no man ask me; ask the Gospel. “But this,” says it, “He said of the Spirit, which they should receive that should believe in Him.” Consequently, the water of the sacrament is one thing: another, the water which betokens the Spirit of God. The water of the sacrament is visible: the water of the Spirit invisible. That washes the body, and betokens that which is done in the soul. By this Spirit the soul itself is cleansed and fed. This is the Spirit of God, which heretics and all that cut themselves off from the Church, cannot have. And whosoever do not openly cut themselves off, but by iniquity are cut off, and being within, whirl about as chaff and are not grain; these have not this Spirit. This Spirit is denoted by the Lord under the name of water: and we have heard from this epistle, “Believe not every spirit;” and those words of Solomon bear witness, “From strange water keep far.” What means, “water”? Spirit. Does water always signify spirit? Not always: but in some places it signifies the Spirit, in some places it signifies baptism, in some places signifies peoples, in some places signifies counsel: thus you find it said in a certain place, “Counsel is a fountain of life to them that possess it.” So then, in various places of the Scriptures, the term “water” signifies various things. Now however by the term water you have heard the Holy Spirit spoken of, not by an interpretation of ours but by witness of the Gospel, where it says, “But this said He of the Spirit, which they should receive that should believe in Him.” If then by the name of water is signified the Holy Spirit, and this epistle says to us, “Believe not every spirit, but prove the spirits, whether they be of God;” let us understand that of this it is said, “From strange water keep far, and from a strange fountain drink not.” What means, “From a strange fountain drink not”? A strange spirit believe not.
Source: Homilies on the First Epistle of John (New Advent)