Eph. 4:4-7 - One…

4 There is one body and one Spirit, just as also you were called in one hope of your calling;
5 one Lord, one faith, one baptism,
6 one God and Father of all who is over all and through all and in all.
7 But to each one of us grace was given according to the measure of Christ’s gift.

The Measure of Christ’s Gift

V. 4 - "There is one body and one Spirit, just as also you were called in one hope of your calling;"

“One body and one Spirit” - See 1 Cor 12:4ff. Despite all the local groups of Christians, and even despite the bickering of denominationalism there is only one body - those who believe and know Christ as their personal Lord and Savior.

That is the bottom line - what will you do with Christ?
The most important question asked in the New Testament is: "Sirs, what must I do to be saved?"
The Answer: "Believe in the Lord Jesus, and you will be saved."

“One Spirit” - There is only one Holy Spirit. It is by the Holy Spirit that we are brought into the body of Christ.

“Called into one hope” - See vs. 1:18. Our one hope is Christ and that He conquered death, and that through Him we too will have life eternal.

V. 5 - “one Lord, one faith, one baptism,”

“One Lord” - See 1 Cor 8:6. The Lord of Lords, King of Kings, the First and the Last. Put any adjective or modifier you want, He is God. (It doesn’t even matter (see note 1) if you believe it or not, because He is. Your belief, or unbelief, cannot change that. If you don’t believe it, God simply is not a reality to you at this time.)  However, the ramifications of belief or unbelief are beyond astronomical.

“One faith” - Not just a mental assent to Christ, not just an emotional experience, but a placing of complete trust and will into the hands of the Lord. Does this mean there is only one way to accept Christ, and all others fail? How does one describe this faith to someone else so the message can be spread? People often use different words, etc., to describe their faith, but generally the end meaning is the same. When we release ourselves to Christ, and accept Himself as Savior; when we turn over every area to Him, then Christ’s working in our lives becomes a reality.
There are a few basics, foundational truths we must acknowledge, see Rom 10:9-10, "that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved; for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation."

“One baptism” - Immersion? Perhaps, but I think not. There is not basis in preceding or following verses to indicate water or immersion baptism. It is the baptism of the Holy Spirit that we are brought into the body of Christ. If this verse speaks of water baptism, then it should say “two baptisms”. Paul is writing about spiritual matters all through these verses. To pop in a reference to water baptism truly seems out of place and character.

V. 6 - “one God and Father of all who is over all and through all and in all.”

These verses (4:4-6) sums up the reasons for unity as believers:

1. One body; 
2. One Spirit; 
3. One calling; 
4. One Lord (Jesus);
5. One faith; 
6. One baptism; 
7. One God (the Father).

If we believe in one God who:

1. Has control of everything with all authority; 
2. Has a part in every action and deed we do; 
3. And is indwelling each of us who believe - 

That should be enough for Christian unity.

V. 7 - “But to each one of us grace was given according to the measure of Christ’s gift.”

Oneness as a body is the desired goal of the believers. Our relationship to God and Jesus is a strictly personal basis. Your father or brother or sister cannot save you, cannot mediate. Your personally must make the decision about Christ (See: Ezek 18:19-23)

But perhaps this verse does not refer to the grace of salvation at all. If verse 7 is tied in with the following verses - through verse 16 - we can see that Paul is beginning a new train of thought dealing with our Christian walk. We have dealt with the position of the Christian in verses 3:1-4:6.

Paul begins a new section on the earthly life of a Christian. He starts that thought with a statement that in essence says, “each of us has a task that has been assigned to us. These tasks are for the simple purpose of discipling the body of Christ. Note these tasks are given only as one becomes a Christian, not before. You have a task, so go do it.”

Paul almost always called his mission to the gospel the Gentiles a “grace” given to him by God (See: 3:2). Grace - defined as “unmerited favor from God” - does not fit very well here. Maybe this fits, “God has given me the privilege of working for Him in this capacity.” This means “in proportion to Christ’s gift.”

What is Christ’s gift?

1. Does this refer to the task given to Christ to be the savior of the world? 
2. Does this refer to the gift of salvation given to each of us by Christ? Or, 
3. Does this refer to a particular gift made available to each of us through Christ over and beyond the initial gift of salvation? 

I think the latter is probably closer to the meaning.

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Note 1: To say it ‘doesn’t matter’ is because He exists whether you believe or not. It does matter spiritually and eternally - the choice determines your eternal existence - in heaven with Him or NOT.