Turretin on the Church Militant and the Church Triumphant

IV. Now as the church is rightly distributed into militant and triumphant, so the distinction between them must be considered not to be essential and specific as to nature but only accidentally as to state and degree. For since there is only one communion of saints and only one body of Christ (Cant. 6:9), as he head by which it is governed is only one and the Spirit by which it is animated only one (Eph. 4:3, 4); so the church in both states is the same, …

V. These two states, however, as so mutually connected by the most wise dispensation of God that they cannot be torn asunder, but necessarily attend and follow each other. Just as no one can be a citizen of the church triumphant who has not given his name before to the militant, nor is anyone crowned in the former with Christ who has not rightly contended with him, so no one is a true member of the church militant here who in his own time will not be carried into the church triumphant; nor is anyone enrolled among believers in grace who will not be received into the choir of the blessed in glory. For whom Christ once received coming to him, he will never cast out (Jn. 6:37), because the bond of our union with him is eternal and indissoluble (adialyton). …

[Francis Turretin, Institutes of Elenctic Theology, 20.XIII.5; v. 3:633-4]

The Church Militant refers to the Invisible Church on earth, fighting sin, the world and the Devil. The Church Triumphant refers to the Church in heaven, who has won over sin, the world and the Devil. Errors like the Federal Vision attempt to separate the two, such that one can be a member of the Church Militant without being a member of the Church Triumphant, due to covenant breaking. However, Scripture teaches that those who are truly saved are the members of the Church Militant, and will never fall away. Thus, 1 Jn. 2:19 states that those who are "covenant breakers" were never truly in the Church in the first place. They broke the covenant externally- that is true, but they were never actually in it.

The Church Militant and the Church Triumphant are thus two sides of the same coin, in different forms. Until Christ comes again, the two forms are present, since we are living in the period between the Inauguration and the Consummation of the Kingdom of God. When Christ returns, the two will be seen as one, and all who believe in Christ will be gathered as the Church Triumphant, praising God and Christ forever and ever.