I am Catholic and have been visiting a Protestant church for the last 2 months. According to Catholicism, did I commit a mortal sin by visiting this church?
According to Catholicism, is repeatedly visiting a Protestant church a mortal sin?
The short answer is: No. Providing that one assists at the sacrifice of the Mass on Sundays and holy days of obligation.
It is both a precept of the Church and Church law that Catholics must worship God on Sunday and Holy Days of Obligation by participating in the Holy Mass. This follows from the fact that in the Mass it is Christ Himself who worships the Father, joining our worship to His. In no other way is it possible to adequately give thanks (eucharistia) to God for the blessings of creation, redemption and our sanctification than by uniting our offerings to that of Jesus Christ Himself. Following the example of the Old Covenant the Church does this weekly, on the day of the Lord's Resurrection.
On Sundays and other holy days of obligation the faithful are bound to participate in the Mass; they are also to abstain from those labors and business concerns which impede the worship to be rendered to God, the joy which is proper to the Lord's Day, or the proper relaxation of mind and body.
The precept of participating in the Mass is satisfied by assistance at a Mass which is celebrated anywhere in a Catholic rite either on the holy day or on the evening of the preceding day.
If because of lack of a sacred minister or for other grave cause participation in the celebration of the Eucharist is impossible, it is specially recommended that the faithful take part in the liturgy of the word if it is celebrated in the parish church or in another sacred place according to the prescriptions of the diocesan bishop, or engage in prayer for an appropriate amount of time personally or in a family or, as occasion offers, in groups of families.
Since a "grave cause" is needed to excuse one from this obligation it would be a serious or mortal sin to willfully skip Mass on Sunday or a Holy Day of Obligation, as the Church has always taught. Reasons such as the necessity to work to support one's family, child care, personal sickness or the care of the sick, necessary travel etc. would excuse a person on a particular occasions. Those who have continuing reason to be excused should consult their pastor.
If a priest is not available in an area and only a Liturgy of the Word or a Communion Service is offered the Mass obligation does not "transfer" to such services. As canon 1248 notes, participation is recommended for the spiritual value, especially if Communion is distributed.
The reason for "visiting" another church may because of several causes. One may be helping a non-Catholic family member to get to their church. One may be doing this for many, many reasons. But the obligation to participate in the sacrifice of the Mass takes precedence and must be fulfilled.
If in doubt, one should seek advice for competent authority such as a deacon, priest or bishop.
I know the case of a deaf woman who after attending a Catholic Mass, she goes to a Protestant service because their services have sign language and no one at her Church (Mass) does sign language. This is done with permission from the diocese where she lives.
Catholics could also be hired by a Protestant Churches to play an organ at their services because of their expertise. Our Catholic parish actually hired a Protestant to play the organ at Mass because of her expertise in playing the organ.
"Visiting" a non-Catholic church is not a mortal sin, providing it remains just that:"visiting". If it is done habitually, one should clarify the situation with one's local pastor or priest.