1 John 1:9

If we receive the witness of men, &c. If we give credit to man's testimony, much more ought we to believe the witness of God concerning Christ. It is greater both in dignity and authority, in truth and certainty. For God infinitely surpasses all men and angels in majesty and veracity. He is the first and supreme Verity, who cannot lie, neither can He deceive, or be deceived. As S. Paul declares (Romans 3:4), "Let God be true, and every man a liar." Moreover, the testimony of the Church, of the Apostles and Prophets, is the testimony of God, for the Church is governed by the Holy Ghost, who is the Spirit of Truth.

Because this is the witness of God, &c. The word because here is not so much causative as explanatory, and means the same as but. The meaning is, But this is God's testimony, because, i.e., which, He has testified, &c., namely, as He testified at the Baptism of Christ, and at many other times, "This is My Beloved Son."