It would depend, I suppose, on what those missionaries are doing. If they are the sole providers of essential health and human charitable services in a particular area, it may be all right to give to them specifically for that purpose. (It would be better though for a Catholic to see if there are Catholic or secular organizations providing the same services and give those organizations financial support.) But it the Evangelical missionaries are “spreading the [non-Catholic] gospel,” particularly in traditionally Catholic countries in which Evangelical missionaries have targeted local Catholics for conversion to Evangelical Protestantism, then for a Catholic to support such a project financially would be deeply problematic.
Just to be clear here, I do not object to Evangelicals sharing their faith with anyone they wish, and they sincerely believe that Catholics need their version of the gospel. It is also true that the Church in those countries needs to take responsibility for providing substantive catechesis to the faithful, instead of complaining of “sheep stealing.” What I object to is the idea of Catholics giving money to Evangelical missionaries for the purpose of making Evangelical converts from Catholicism in traditionally Catholic countries.
