I believe the Catholic faith is true. Suppose I wish to draw closer spiritually to non-Catholic Christians. What should I do? Well, the kind of spiritual closeness that I should seek is closeness in Christ. To be closer to non-Catholic Christians, then, I need to be closer to Christ. But if the Catholic faith is true, then I come closer to Christ in being faithful to the Catholic faith. Therefore, I come closer in Christ to non-Catholic Christians by being faithful to the Catholic faith. This is counterintuitive but hard to deny (except by denying the truth of the Catholic faith). If this lesson is learned, then many of the distortions of ecumenism will be averted.
Here is a different, and even more pointed, way of looking at it. According to Vatican II, non-Catholic Christians have an imperfect union with the Catholic Church through baptism, and I think it is a very plausible theological reading of this that I am united in the appropriate way with these non-Catholic Christians precisely in and through their and my union with the Catholic Church. If, then, I am to grow closer to them spiritually in the appropriate way, I need to cleave more closely to the Catholic Church. Furthermore, non-Catholic Christians are united spiritually to one another in the appropriate way only insofar as they are united to Christ. And they are united to Christ insofar as they are united to the Catholic Church, the body of Christ. The unity of non-Catholic Christians, thus, depends on their union with the Catholic Church.