I think you will notice that most Protestants like to brag that their faith is "based upon the Bible", "scriptural", "the one true interpretation". And, that is the problem. First of all, they're not based upon the Bible. Most Protestants reject "faith, without good works, is dead". They say only faith is required. Secondly, most protestants reject the very-biblical truths, such as Purgatory, the Immaculate Conception, the perpetual Virginity of our Blessed Virgin, and even some go so far as to reject the Divinity of Our Lord and Saviour, Jesus Christ.
It is truly a shame, since so many souls are being led away from the truth, but these mega-church Preachers on the television, by the local Jehovah's Witnesses, by the SDA church down the road. Why? To be led away, these people obviously must not have had a strong faith to begin with. Or, is that not the case?
Maybe, it's because their Parish Priest (or their "qualified lay homilist")had taught the poor soul that all religions and beliefs are equal, that they all lead to God. Therefore, the poor soul thinks "well, I feel better and like the ___________'s beliefs better, my Priest said they're all equal. I'll give 'em a try". We know that the idea of "all faiths being equal", is a blatant lie. I fail to see how there could be more than one way to God. Why would He require certain beliefs of one Church, then entire different teachings in another. Answer, there wouldn't and there isn't. In essence, it defeats the purpose of having Truth.
Protestants lack the fullness of Truth
The is truth, the only Truth, and always shall be, always has been. The Truth is most holy and divine, and it comes from God most high himself, and is slowly revealed to us by Holy Mother Church. This is the joy that comes from being Catholic. This is the great joy experienced by all the Saints. To imitate Christ, with the Church. To imitate the Blessed Virgin, Mother of God!
One of the most effective ways to bring souls back into the Church, is by prayer. Pray the Holy Rosary for all souls, and pray to the Saints, your patron Saint. Meditate upon the passion of Our Lord, and assist at Mass fervently.