If by "Catholic" you mean the body of believers in Christ, and Christ alone, then you are correct and Satan cannot own the ekklesia.
However, if by "catholic" you mean the Roman Catholic church, and there is a difference, then there can be Satanic infiltration in that denomination. And from what is posted in the article that does indeed appear to be the case.
Your comment is like saying the Church founded upon the Apostles was a Satanic Church because they all either betrayed, denied, or abandoned Him --- except for one, John, who stayed at the foot of the Cross with Jesus' sorrowful mother.
A couple of clarifications and corrections:
All the disciples abandoned Him at the time of His arrest.
56“But all this has taken place to fulfill the Scriptures of the prophets.” Then all the disciples left Him and fled. Matthew 26:56 NASB
The ekklesia is built upon:
18for through Him we both have our access in one Spirit to the Father. 19So then you are no longer strangers and aliens, but you are fellow citizens with the saints, and are of God’s household,
20having been built on the foundation of the apostles and prophets, Christ Jesus Himself being the corner stone,
21in whom the whole building, being fitted together, is growing into a holy temple in the Lord, 22in whom you also are being built together into a dwelling of God in the Spirit. Ephesians 2:18-22 NASB
As a visible, God-ordained body, a parish or congregation or other "institutional" ekklesia will be very much this sort of mixture. This should sadden us all but should surprise nobody.
As for St. John, we don't actually know if he panicked and dashed off with the others or not--- possibly he just went to get Jesus' mother --- but either way, he must have returned promptly, because we know he was there with Mary, at the Lord's crucifixion to the end. At that point, the Church Loyal was indeed very small at the foot of the Cross: very small.