Unity requires three things at least.
- unity in doctrine
- unity in sacraments
- unity in governance
Protestants vary in their views of all three so unity with all the various Protestant groups hasn't happened among the Protestants and one cannot expect unity with Catholics in that state.
The simplicity of abiding in Christ <---His body, the Catholic/Universal Church:
Unity in the Holy Spirit = Love God with all your heart mind and soul and love your neighbor as yourself.
John 13:33-35
33 Little children, yet a little while I am with you. Ye shall seek me: and as I said unto the Jews, Whither I go, ye cannot come; so now I say to you.
34 A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another.
35 By this shall all men know that ye are my disciples, if ye have love one to another.
1 John 2:5-10
5 But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him.
6 He that saith he abideth in him ought himself also so to walk, even as he walked.
7 Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning.
8 Again, a new commandment I write unto you, which thing is true in him and in you: because the darkness is past, and the true light now shineth.
9 He that saith he is in the light, and hateth his brother, is in darkness even until now.
10 He that loveth his brother abideth in the light, and there is none occasion of stumbling in him.