Does the Catholic Church teach that is a sin to attend another Christian Church that is truly focused on the Bible?
If it is OK to attend another Christian Church can they take the Holy Bread and Grape Juice? Is this the same as the Eucharist?
The Catholic Church requires attendance at a Catholic Mass on Sunday's and Holy days of Obligation. Attending another Christian Church does not meet this obligation.
The precept of the Church specifies the law of the Lord more precisely: "On Sundays and other holy days of obligation the faithful are bound to participate in the Mass."117 "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 you do attend another Christian Church you should not take the bread. Other Christian churches do not have valid Holy Orders and usually do not believe in the Real Presence of Christ in the Eucharist as Catholics do.
Ecclesial communities derived from the Reformation and separated from the Catholic Church, "have not preserved the proper reality of the Eucharistic mystery in its fullness, especially because of the absence of the sacrament of Holy Orders." It is for this reason that Eucharistic intercommunion with these communities is not possible for the Catholic Church. However these ecclesial communities, "when they commemorate the Lord's death and resurrection in the Holy Supper . . . profess that it signifies life in communion with Christ and await his coming in glory."